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
@@ -32,7 +32,7 @@ WARNFLAGS	:= -Wall -Wno-unknown-warning-option -Wno-c9
 CXXFLAGS	?= -O3 -flto -DNDEBUG
 # Non-overridable CXXFLAGS
 REALCXXFLAGS	:= ${CXXFLAGS} ${WARNFLAGS} -std=c++2a -I include \
-		   -D_POSIX_C_SOURCE=200809L -fno-exceptions -fno-rtti
+		   -fno-exceptions -fno-rtti
 # Overridable LDFLAGS
 LDFLAGS		?=
 # Non-overridable LDFLAGS
