Accept CMake args as env vars for Python setup

README.md

@@ -23,7 +23,7 @@ Doing so will make some things run faster.
 
 Finally, Visual Studio users should usually do everything in 64bit mode.  By default Visual Studio is 32bit, both in its outputs and its own execution, so you have to explicitly tell it to use 64bits.  Since it's not the 1990s anymore you probably want to use 64bits.  Do that with a cmake invocation like this:
 ```bash
-cmake .. -G "Visual Studio 14 2015 Win64" -T host=x64 
+cmake .. -G "Visual Studio 14 2015 Win64" -T host=x64
 ```
 
 ## Compiling your own C++ programs that use dlib
@@ -48,6 +48,10 @@ cd dlib
 pip install .
 ```
 
+It's possible to change build settings by passing parameters to `setup.py` or `DLIB_*` environment variables.
+For example, setting the environment variable `DLIB_NO_GUI_SUPPORT` to `ON` will add the cmake option
+`-DDLIB_NO_GUI_SUPPORT=ON`.
+
 
 ## Running the unit test suite
 
@@ -69,4 +73,3 @@ This library is licensed under the Boost Software License, which can be found in
 ## dlib sponsors
 
 This research is based in part upon work supported by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA) under contract number 2014-14071600010. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of ODNI, IARPA, or the U.S. Government.
-

setup.py

@@ -11,7 +11,7 @@
 To build and install:
     python setup.py install
 To upload the source distribution to PyPi
-    python setup.py sdist 
+    python setup.py sdist
     twine upload dist/dlib-*.tar.gz
 To exclude certain options in the cmake config use --no:
     for example:
@@ -25,21 +25,21 @@
     --set: set arbitrary cmake options e.g. --set CUDA_HOST_COMPILER=/usr/bin/gcc-6.4.0
            passes -DCUDA_HOST_COMPILER=/usr/bin/gcc-6.4.0 to CMake.
 """
+import errno
+import logging
+import multiprocessing
 import os
+import platform
 import re
-import sys
-import errno
-import stat
 import shutil
-import platform
+import stat
 import subprocess
-import multiprocessing
+import sys
 from math import floor
 
 from packaging.version import Version, parse as parse_version
 from setuptools import find_packages, setup, Extension
 from setuptools.command.build_ext import build_ext
-import logging
 
 logging.basicConfig(level=logging.INFO)
 log = logging.getLogger("setup")
@@ -89,8 +89,6 @@ def get_extra_cmake_options():
 
     return _cmake_extra_options, _clean_build_folder
 
-cmake_extra_options,clean_build_folder = get_extra_cmake_options()
-
 
 class CMakeExtension(Extension):
     def __init__(self, name, sourcedir=''):
@@ -143,7 +141,7 @@ def get_cmake_version(self):
     packager delete it and install an official cmake.
 
     More generally, cmake is not installed if when you open a terminal window
-    and type 
+    and type
        cmake --version
     you get an error.  So you can use that as a very basic test to see if you
     have cmake installed.  That is, if cmake --version doesn't run from the
@@ -157,7 +155,7 @@ def get_cmake_version(self):
 ================================================================================
 ================================================================================
 ================================================================================
-""") 
+""")
             sys.exit(1)
         return re.search(r'version\s*([\d.]+)', out.decode()).group(1)
 
@@ -172,13 +170,19 @@ def run(self):
             self.build_extension(ext)
 
     def build_extension(self, ext):
+        # Build CMake args based on default settings, environment variables and CLI parameters
         extdir = os.path.abspath(os.path.dirname(self.get_ext_fullpath(ext.name)))
-
-        cmake_args = ['-DCMAKE_LIBRARY_OUTPUT_DIRECTORY=' + extdir,
-                      '-DPYTHON_EXECUTABLE=' + sys.executable,
-                      '-DDLIB_USE_FFMPEG=OFF',]
-
-        cmake_args += cmake_extra_options 
+        cmake_extra_options, clean_build_folder = get_extra_cmake_options()
+        cmake_args_dict = {
+            'CMAKE_LIBRARY_OUTPUT_DIRECTORY': extdir,
+            'DLIB_USE_FFMPEG': 'OFF',
+            'PYTHON_EXECUTABLE': sys.executable,
+        }
+        for key, value in os.environ.items():
+            if key.startswith("DLIB_"):
+                cmake_args_dict[key] = value
+        cmake_args = ['-D' + key + '=' + str(value) for key, value in cmake_args_dict.items()]
+        cmake_args += cmake_extra_options
 
         cfg = 'Debug' if self.debug else 'Release'
         build_args = ['--config', cfg]
@@ -188,7 +192,7 @@ def build_extension(self, ext):
             if sys.maxsize > 2**32:
                 cmake_args += ['-A', 'x64']
             # Do a parallel build
-            build_args += ['--', '/m'] 
+            build_args += ['--', '/m']
         else:
             cmake_args += ['-DCMAKE_BUILD_TYPE=' + cfg]
             # Do a parallel build
@@ -220,9 +224,9 @@ def num_available_cpu_cores(ram_per_build_process_in_gb):
     elif 'CMAKE_BUILD_PARALLEL_LEVEL' in os.environ and os.environ['CMAKE_BUILD_PARALLEL_LEVEL'].isnumeric():
         return int(os.environ['CMAKE_BUILD_PARALLEL_LEVEL'])
     try:
-        mem_bytes = os.sysconf('SC_PAGE_SIZE') * os.sysconf('SC_PHYS_PAGES')  
+        mem_bytes = os.sysconf('SC_PAGE_SIZE') * os.sysconf('SC_PHYS_PAGES')
         mem_gib = mem_bytes/(1024.**3)
-        num_cores = multiprocessing.cpu_count() 
+        num_cores = multiprocessing.cpu_count()
         # make sure we have enough ram for each build process.
         mem_cores = int(floor(mem_gib/float(ram_per_build_process_in_gb)+0.5));
         # We are limited either by RAM or CPU cores.  So pick the limiting amount