Fix compilation issue on OSX

[SanderMertens]

On OSX the httpserver fails with this compiler error:

In file included from src/civetweb.c:221:
/usr/include/time.h:177:5: error: conflicting types for '_civet_clock_gettime'
int clock_gettime(clockid_t __clock_id, struct timespec *__tp);
    ^
src/civetweb.c:215:23: note: expanded from macro 'clock_gettime'
#define clock_gettime _civet_clock_gettime
                      ^
src/civetweb.c:159:1: note: previous definition is here
_civet_clock_gettime(int clk_id, struct timespec *t)

The file originates from the civetweb/civetweb GitHub repository, where this issue has not been solved (when I last checked). A fix on this repository should ideally be ported back to the civetweb repository as well.