GLdispatch: Add a build option to disable entrypoint patching.

Added a --disable-entrypoint-tracking configure option and an
'entrypoint-patching' meson option to disable libGLdispatch's entrypoint
patching at build time.

If entrypoint patching is disabled, then it #ifdef's out the mprotect call, and
acts as if mprotect had failed, which causes libGLdispatch to skip trying to
perform any patching.

Fixes glvnd/libglvnd#217
2 jobs for entrypoint-patch-build-option in 1 minute and 42 seconds