libcdecl: Prefer __inline in public header with GCC.

libcdecl: Prefer __inline in public header with GCC.

If the library is built using GCC with default options, the configure
tests for inline will determine it is supported, and the installed
headers will use it.

But that doesn't mean the user is running the compiler in the exactly
the same mode when including the headers.  If they then use GCC in
strict C89 mode, "inline" is not recognized as a keyword and compilation
will fail.  We can avoid this problem by just always using __inline on
GCC, with a fallback to the configure-detected result.