Replace TRUE/FALSE with IS_TRUE/IS_FALSE on macros

[pafuent]

This fix projects/compilers that has defined TRUE as a macro. As an example, using cpp_mock with mingw will result on mocked method names not being identified as defined types when the number of parameters where one or zero.
That was caused because DEFINE_EXISTS macro used TRUE which was expanded to 1, and that being concatenated to MOCK_METHOD_SINGLE_. Because MOCK_METHOD_SINGLE_1 was not defined as a macro the preprocessed code was invalid.

If you compile this with mingw you can reproduce the issue:

#include "cpp_mock.h"

struct SomeInterface {
    virtual int foo(const int&, float, std::string) = 0;
    virtual int bar(std::string) = 0;
};

struct SomeInterfaceMock : public SomeInterface {
  MockMethod(int, foo, (const int&, float, std::string));
  MockMethod(int, bar, (std::string));
};

int main() {
  SomeInterfaceMock mock;

  When(mock.foo).With(1, 3.14, "foo123").Return(42);
  assert(mock.foo(1, 3.14, "foo123") == 42);

  mock.bar("bar123");
  Verify(mock.bar).With("bar123").Times(1);
}