Mesa removed that library in 027ccd963b1f8f288bef4224aedcddc1557e4f8a,
so this API can be removed from GLFW too.
Mesa has provided an alternative path for years, through the
EGL_MESA_platform_surfaceless EGL extension, and it also supports
selecting the software rasterizer through the EGL_EXT_platform_device
EGL extension, neither of which are supported by GLFW yet.
I kept the external API the same, notably the GLFW_OSMESA_CONTEXT_API
define is still present, but will error at runtime.
The original MinGW distribution appears to no longer be maintained and
should not be used. Anyone still using MinGW should consider switching
to the MinGW-w64 fork or another actively maintained toolchain.
MinGW-w64 supports 64-bit binaries and provides much newer compilers and
Win32 headers.
Fixes#2540
