[Config.h] Make all the defines in config.h be override-able by the build system

[JeffM2501]

Per issue #4411 this PR makes all the config defines first check if they have been set by the build system, and doesn't just check for the define, but instead check the value.
This allows the config.h values to be individually set by the build system.

[Peter0x44]

Are you sure this is enough? Anything you pass on the command line will still get redefined in config.h itself.
This lets you turn on extra things, but not turn off things, and not change the value of anything.

[JeffM2501]

Are you sure this is enough? Anything you pass on the command line will still get redefined in config.h itself.

Yes because config.h now checks if the label is not already defined first before defining it, so things on the command line will be defined before the preprocessor runs and config.h will not replace them. This ensures that all things are still defined. I have tested this in msvc and get expected results, I can build raylib with a define in the build system only to disable screenshots. Please test it in other systems.

What this won't do is disable things at runtime, since it's all still baked into the compile with preprocessor directives.

[Peter0x44]

I missed the change to config.h because GitHub collapsed it, sorry! I will test this with gcc/make on Linux later.

[Peter0x44]

I tested this with:
make CUSTOM_CFLAGS="-DSUPPORT_FILEFORMAT_PNG=0" and I was then unable to load PNGs as expected
LGTM