build: Add options to override subproject paths

[K1ngst0m]

The PR adds two Meson string options for developers—such as distro packagers—who often have up-to-date or patched libraries installed system-wide or in custom paths:

• -Dglm_include_dir=…
• -Dstb_include_dir=…

If either option is non-empty, Meson will use the specified include path; otherwise, it falls back to the wrapped subproject setup.

Usage example

meson setup builddir \
  -Dglm_include_dir=/path/to/include/glm \
  -Dstb_include_dir=/path/to/include/stb \

[K900]

Thanks, this will work great for our purposes.

[K900]

@misyltoad bump? (sorry...)