Better integration of libevent in build system.

Since event.h is not part of tinc, we include it in have.h were all other
system header files are included.  We also ensure -levent comes before -lgdi32
when compiling with MinGW, apparently it doesn't work when the order is
reversed.

have.h

@@ -37,6 +37,7 @@
 #else
 #define WINVER WindowsXP
 #endif
+#define WIN32_LEAN_AND_MEAN
 #include <w32api.h>
 #include <windows.h>
 #include <ws2tcpip.h>
@@ -179,4 +180,8 @@
 #include <netinet/if_ether.h>
 #endif
 
+#ifdef HAVE_EVENT_H
+#include <event.h>
+#endif
+
 #endif /* __TINC_SYSTEM_H__ */