This fixes gamepad mappings being included in GLFW binaries that would
then never use them because they were for a different platform.
(cherry picked from commit 0c90e1a33f)
Inclusion of internal headers is already both centralized and follows
strict rules. Inclusion guards are both an unneccessary maintenance
burden and may hide inclusion order bugs.
Use platform prefix for files specific to that platform AND that have no
credible alternative API on that platform.
The exception is WinMM, which will be replaced before 3.2.