Make compile with GraphViz >= 2.36.0

Fixes #70

Couldn't find a better solution. Unfortunately GraphViz headers actually use a lot more of the HAVE_FOO_H defines, in theory we'd have to define *all* of them in FindGraphviz.cmake.

Or is there a way to automatically check for common STL headers inCMake?

Sample from Graphviz's Makefiles:
graphs/undirected/Makefile
396:TCL_DEFS = -DPACKAGE_NAME=\"tcl\" -DPACKAGE_TARNAME=\"tcl\" -DPACKAGE_VERSION=\"8.6\" -DPACKAGE_STRING=\"tcl\ 8.6\" -DPACKAGE_BUGREPORT=\"\" -DSTDC_HEADERS=1 -DHAVE_SYS_TYPES_H=1 -DHAVE_SYS_STAT_H=1 -DHAVE_STDLIB_H=1 -DHAVE_STRING_H=1 -DHAVE_MEMORY_H=1 -DHAVE_STRINGS_H=1 -DHAVE_INTTYPES_H=1 -DHAVE_STDINT_H=1 -DHAVE_UNISTD_H=1 -DHAVE_LIMITS_H=1 -DHAVE_SYS_PARAM_H=1 -DUSE_THREAD_ALLOC=1 -D_REENTRANT=1 -D_THREAD_SAFE=1 -DHAVE_PTHREAD_ATTR_SETSTACKSIZE=1 -DHAVE_PTHREAD_ATFORK=1 -DTCL_THREADS=1 -DTCL_CFGVAL_ENCODING=\"iso8859-1\" -DHAVE_ZLIB=1 -DMODULE_SCOPE=extern -DHAVE_CAST_TO_UNION=1 -DTCL_SHLIB_EXT=\".so\" -DNDEBUG=1 -DTCL_CFG_OPTIMIZED=1 -DTCL_TOMMATH=1 -DMP_PREC=4 -D_LARGEFILE64_SOURCE=1 -DTCL_WIDE_INT_IS_LONG=1 -DHAVE_GETCWD=1 -DHAVE_MKSTEMP=1 -DHAVE_OPENDIR=1 -DHAVE_STRTOL=1 -DHAVE_WAITPID=1 -DHAVE_GETNAMEINFO=1 -DHAVE_GETADDRINFO=1 -DHAVE_FREEADDRINFO=1 -DHAVE_GAI_STRERROR=1 -DHAVE_STRUCT_ADDRINFO=1 -DHAVE_STRUCT_IN6_ADDR=1 -DHAVE_STRUCT_SOCKADDR_IN6=1 -DHAVE_STRUCT_SOCKADDR_STORAGE=1 -DHAVE_GETPWUID_R_5=1 -DHAVE_GETPWUID_R=1 -DHAVE_GETPWNAM_R_5=1 -DHAVE_GETPWNAM_R=1 -DHAVE_GETGRGID_R_5=1 -DHAVE_GETGRGID_R=1 -DHAVE_GETGRNAM_R_5=1 -DHAVE_GETGRNAM_R=1 -DHAVE_GETHOSTBYNAME_R_6=1 -DHAVE_GETHOSTBYNAME_R=1 -DHAVE_GETHOSTBYADDR_R_8=1 -DHAVE_GETHOSTBYADDR_R=1 -DHAVE_TERMIOS_H=1 -DHAVE_SYS_IOCTL_H=1 -DHAVE_SYS_TIME_H=1 -DTIME_WITH_SYS_TIME=1 -DHAVE_GMTIME_R=1 -DHAVE_LOCALTIME_R=1 -DHAVE_MKTIME=1 -DHAVE_TM_GMTOFF=1 -DHAVE_TIMEZONE_VAR=1 -DHAVE_STRUCT_STAT_ST_BLOCKS=1 -DHAVE_STRUCT_STAT_ST_BLKSIZE=1 -DHAVE_BLKCNT_T=1 -DHAVE_INTPTR_T=1 -DHAVE_UINTPTR_T=1 -DHAVE_SIGNED_CHAR=1 -DHAVE_LANGINFO=1 -DHAVE_MKSTEMPS=1 -DHAVE_FTS=1 -DHAVE_SYS_IOCTL_H=1 -DTCL_UNLOAD_DLLS=1 -DHAVE_CPUID=1