Change detection for the gnu_inline attribute.

According to the GCC documentation:
"This attribute is available in GCC 4.1.3 and later. It is available
if either of the preprocessor macros GNUC_GNU_INLINE or
GNUC_STDC_INLINE are defined."

We don't keep the gcc granularity up to the minor number so it's
better to use the documented way. Current clang defines both
macros.

Reference:
https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes