Using own version of timegm when applicable.

Some systems come without timegm function in standard library. Taskwarrior
already has its own implementation defined in util.c. However, this has
never been used as util.h has not been included.

src/ISO8601.cpp

@@ -28,6 +28,7 @@
 #include <sstream>
 #include <stdlib.h>
 #include <Lexer.h>
+#include <util.h>
 #include <ISO8601.h>
 #include <Date.h>
 

src/Nibbler.cpp

@@ -31,6 +31,7 @@
 #include <inttypes.h>
 #include <Lexer.h>
 #include <Nibbler.h>
+#include <util.h>
 #ifdef NIBBLER_FEATURE_DATE
 #include <Date.h>
 #endif