Fix stack overflow that can occur in libevhtp

libevhtp allocates a stack based on data length
when C99 is detected at compile time. There are
no checks to verify that the stack is big enough
which can cause a stack overflow.

Adding EVHTP_HAS_C99=false at compile time changes
this behavior by allocate to a buffer which has
proper checks in place.

More information about this bug can be found at:
Yellow-Camper/libevhtp#118
haiwen/seafile#1928

MFH:		2019Q3

www/libevhtp/Makefile

@@ -2,7 +2,7 @@
 
 PORTNAME=	libevhtp
 PORTVERSION=	1.2.16
-PORTREVISION=	3
+PORTREVISION=	4
 CATEGORIES=	www
 
 MAINTAINER=	ultima@FreeBSD.org
@@ -19,7 +19,8 @@ USE_GITHUB=	yes
 GH_ACCOUNT=	criticalstack
 
 CMAKE_ARGS=	-DCMAKE_INCLUDE_PATH:PATH=include/event2 \
-		-DCMAKE_LIBRARY_PATH:PATH=lib/event2
+		-DCMAKE_LIBRARY_PATH:PATH=lib/event2 \
+		-DEVHTP_HAS_C99:BOOL=FALSE
 
 PLIST_SUB=	PORTVERSION=${PORTVERSION}