* parser: Fail gracefully if a nonexistent file is passed on the command line

Before, if a nonexistent file was passed to LoadFileText(), it would
return NULL, and the parser would happily dereference it.

* parser: Refactor Makefile and update the path to easings.h (now reasings.h)

Before, the `make all` target would simply segfault, see 0a679d79
Now, if a file in the `make all` target doesn't exist, make will write
an error.

Individual API files can be generated likeso, provided the header file
the target depends on exists:

FORMAT=JSON EXTENSION=json make raygui_api.json

In order for the `make all` target to succeed, raygui.h, physac.h and
rmem.h need to be added to the correct directory.

* Add support for truncating parser input

* Remove RLAPI from implementations in rlgl.h

* added defines to parser

* added value and description for defines in parser

* parser: fixed lua defines output, fixed makefile for raygui, added LONG type for defines

* parser defines: remove postfix for LONG/FLOAT, support for hexadecimal consts, convert hex ints to decimal for JSON output

* removed defines from raylib_apis