CMake glad_add_library(): MERGE trying to build non-existent file
Example config (based on example/src/egl_x11)
```
glad_add_library(glad_egl_15_gles2_20 REPRODUCIBLE MERGE LOADER API egl=1.5 gl:core=3.3 gles2=2.0)
```

The goal (which seems to work) is to merge OpenGL 3.3 and GLES2 symbols.
This means that glad only produces a single `gl.c` for the two API, but the CMake machinery tries to build `gles2.c` as well:

```
Writing /Users/kintel/code/OpenGL/glad/example/c/egl_x11/gladsources/glad_egl_15_gles2_20/args.txt
[ 28%] Building C object CMakeFiles/glad_egl_15_gles2_20.dir/gladsources/glad_egl_15_gles2_20/src/egl.c.o
[ 42%] Building C object CMakeFiles/glad_egl_15_gles2_20.dir/gladsources/glad_egl_15_gles2_20/src/gl.c.o
[ 57%] Building C object CMakeFiles/glad_egl_15_gles2_20.dir/gladsources/glad_egl_15_gles2_20/src/gles2.c.o
clang: error: no such file or directory: '/Users/kintel/code/OpenGL/glad/example/c/egl_x11/gladsources/glad_egl_15_gles2_20/src/gles2.c'
clang: error: no input files
```