[build] [web] Fix examples Makefile for PLATFORM_WEB

[asdqwe]

eb04154 changed emmake make to mingw32-make for PLATFORM_WEB. However, mingw32-make isn't always present (e.g.: Linux hosts), which leads to these:

~/raylib-master/examples$ make PLATFORM=PLATFORM_WEB core/core_basic_window
find . -type f -executable -delete
rm -fv *.o
Cleaning done
mingw32-make -f Makefile.Web core/core_basic_window
make: mingw32-make: No such file or directory
make: *** [Makefile:660: core/core_basic_window] Error 127

Since emmake is usually provided by Emscripten's emsdk, this PR checks if emmake is present. If it is, uses it. If not, falls back to mingw32-make.

[raysan5]

@asdqwe Merging... let me test it on Windows... here the result launching the examples/Makefile from Windows to call examples/Makefile.Web (also modified):

mingw32-make models/models_gpu_skinning PLATFORM=PLATFORM_WEB -B
Process started (PID=20276) >>>
process_begin: CreateProcess(NULL, type emmake, ...) failed.
Makefile:200: pipe: No error
mingw32-make -f Makefile.Web models/models_gpu_skinning
make[1]: Entering directory 'C:/GitHub/raylib/examples'
emcc -o models/models_gpu_skinning.html models/models_gpu_skinning.c -Wall -std=c99 -D_DEFAULT_SOURCE -Wno-missing-braces -Wunused-result -Os -I. -I../src -I../src/external -L. -L../src -L../src -sUSE_GLFW=3 -sASYNCIFY -sEXPORTED_RUNTIME_METHODS=ccall --shell-file ../src/shell.html ../src/libraylib.a -DPLATFORM_WEB -sTOTAL_MEMORY=67108864 \
    --preload-file models/resources/models/gltf/greenman.glb@resources/models/gltf/greenman.glb \
    --preload-file models/resources/shaders/glsl100/skinning.vs@resources/shaders/glsl100/skinning.vs \
    --preload-file models/resources/shaders/glsl100/skinning.fs@resources/shaders/glsl100/skinning.fs
make[1]: Leaving directory 'C:/GitHub/raylib/examples'

It works but note that, for some reason emmake is not properly detected on Windows...

[raysan5]

@asdqwe Potential related issue: https://github.com/emscripten-core/emscripten/blob/main/emmake.bat#L42

[raysan5]

@asdqwe Probably a good idea to add the check, so we avoid a first line failure.