timegm is not declared on Haiku

[Begasus]

Adding +#define _DEFAULT_SOURCE at

libshared/src/Datetime.cpp

Line 27 in 9a5f24e

#include <cmake.h>

fixes it.
Passing -DCMAKE_CXX_FLAGS="-D_STANDARD_SOURCE" when building taskwarrior doesn't seem to pass it over to libshared