workaround for vasprintf bug in openbsd headers when
POSIX_C_SOURCE=200809L is defined

see:
https://github.com/gbdev/rgbds/issues/1111
https://marc.info/?l=openbsd-tech&m=161833545600505&w=2
https://marc.info/?l=openbsd-bugs&m=157758838031146&w=2

Index: Makefile
--- Makefile.orig
+++ Makefile
@@ -42,7 +42,7 @@ CXXFLAGS	?= -O3 -flto -DNDEBUG
 REALCFLAGS	:= ${CFLAGS} ${WARNFLAGS} -std=gnu11 -I include \
 		   -D_POSIX_C_SOURCE=200809L -D_ISOC11_SOURCE
 REALCXXFLAGS	:= ${CXXFLAGS} ${WARNFLAGS} -std=c++17 -I include \
-		   -D_POSIX_C_SOURCE=200809L -fno-exceptions -fno-rtti
+		   -fno-exceptions -fno-rtti
 # Overridable LDFLAGS
 LDFLAGS		?=
 # Non-overridable LDFLAGS