Allow to predefine FORCE_INLINE macro.

Allow to predefine FORCE_INLINE macro.

FORCE_INLINE macro is defined based on the compiler used. When using
gcc, it will include "__attribute__((always_inline))" forcing gcc to
always inline all the functions marked as FORCE_INLINE. However, this
can cause a performance degradation of about 15%.

This patch allows to set the FORCE_INLINE macro from the compiler
command line to either "static" or "static inline" giving allowing it
to inline functions as needed when performing optimizations.