Some header files use __BEGIN_DECLS without including sys/cdefs.h.
This causes issues for C code that compiles against these headers,
which may occur with Ports.
This adds a stub for fnmatch and the following defined values:
- FNM_PATHNAME
- FNM_NOESCAPE
- FNM_PERIOD
- FNM_FILE_NAME
- FNM_LEADING_DIR
- FNM_CASEFOLD
- FNM_EXTMATCH