[CMake] parsing config.h files and setting cmake variables accordingly

[Clemens Arth]

Hi,

I've got a  question concerning  string processing in cmake. Our 
software framework consists of multiple libraries and has many different 
features to be enabled or disabled using #defines. 
For example, one option is to compile with OpenGL or with OpenGL ES 
support. Thus in a config.h file, one of two variables is valid to be 
#defined, USE_OPENGL or USE_OPENGLES. Depending on the variable defined 
cmake should link against a specific set of libraries.

Currently determining which feature is set works the following way in my 
CMakeLists.txt:

Code:
# check for the configuration and set the corresponding GL/GLES 
libraries accordingly
FILE(READ ${LIB_SOURCE_DIR}/include/config.h CURRENT_CONFIG)
STRING(REGEX MATCH "\#define USE_OPENGLES" GLES_IS_SET ${CURRENT_CONFIG})
STRING(REGEX MATCH "\#define USE_OPENGL" GL_IS_SET ${CURRENT_CONFIG})
IF("#define USE_OPENGLES" STREQUAL "${GLES_IS_SET}")
    MESSAGE("GLES config!")
ELSE("#define USE_OPENGLES" STREQUAL "${GLES_IS_SET}")
    IF("#define USE_OPENGL" STREQUAL "${GL_IS_SET}")
        MESSAGE("GL config!")
    ELSE("#define USE_OPENGL" STREQUAL "${GL_IS_SET}")
        MESSAGE("Error! USE_GL or USE_GLES must be defined!")
    ENDIF("#define USE_OPENGL" STREQUAL "${GL_IS_SET}")
ENDIF("#define USE_OPENGLES" STREQUAL "${GLES_IS_SET}")


Note that this is really a bad hack. First, if GLES_IS_SET is set 
,GL_IS_SET is also set automatically. Second, if by accident the string 
does not exactly match the content (an additional <space>, or there is a 
second variable, for example called USE_OPENGL_EXTRAS), everything gets 
really weird or fails at all. Finally, a problem is that cmake does not 
actually notice if I changed the config.h file, so there should be an 
option to mark the configuration as dirty if the config.h file is 
altered - this is a problem which must not necessarily be solved, but 
maybe there is a simple solution to this.

Can someone give me some tips how to improve my really ugly solution?

Thanks and best regards
Clemens