Fixed duplicate case value when compiling libxml2 on AIX

This is the error we got:
```
00:01:07 xmlIO.c: In function 'xmlIOErr':
00:01:07 xmlIO.c:502:9: error: duplicate case value
00:01:07          case ENOTEMPTY: code = XML_IO_ENOTEMPTY; break;
00:01:07          ^~~~
00:01:07 xmlIO.c:439:9: error: previously used here
00:01:07          case EEXIST: code = XML_IO_EEXIST; break;
00:01:07          ^~~~
```

It appears that the `ENOTEMPTY` & `EEXITS` are defined as the same
value. Defining `_LINUX_SOURCE_COMPAT` should give them unique values.

```C
/*
 * AIX returns EEXIST where 4.3BSD used ENOTEMPTY;
 * but, the standards insist on unique errno values for each errno.
 * A unique value is reserved for users that want to code case
 * statements for systems that return either EEXIST or ENOTEMPTY.
 */
```

Signed-off-by: Lars Erik Wik <lars.erik.wik@northern.tech>
(cherry picked from commit 794e1fd)

Author: larsewi

deps-packaging/libxml2/cfbuild-libxml2.spec

@@ -30,7 +30,7 @@ then
     chmod a+x configure
 fi
 ./configure --prefix=%{prefix} --without-python --enable-shared --disable-static --with-zlib=%{prefix} \
-    CPPFLAGS="-I%{prefix}/include" \
+    CPPFLAGS="-I%{prefix}/include -D_LINUX_SOURCE_COMPAT" \
     LD_LIBRARY_PATH="%{prefix}/lib" LD_RUN_PATH="%{prefix}/lib"
 
 %build
@@ -83,5 +83,3 @@ CFEngine Build Automation -- libxml2 -- development files
 %prefix/lib/pkgconfig
 
 %changelog
-
-