Allow compiling on OpenBSD

Thanks rain1 for the suggestion for this patch. Unfortunately, it came bundled
with other changes which couldn't be accepted, so here is the import-able part.

config.mk

@@ -5,9 +5,10 @@ WARN_CFLAGS = -Wall -Wextra -pedantic -D_XOPEN_SOURCE=700
 DBG_CFLAGS  = -g
 DBG_LDFLAGS = -g
 
-FT_CFLAGS  = $(shell freetype-config --cflags)
-FT_LDFLAGS = $(shell freetype-config --libs)
+FT_CFLAGS  = `freetype-config --cflags`
+FT_LDFLAGS = `freetype-config --libs`
 
+XLIB_CFLAGS  = -I/usr/X11R6/include/
 XLIB_LDFLAGS = -lX11 -lXrender
 
 CFLAGS  += -std=c99 ${XLIB_CFLAGS} ${FT_CFLAGS} ${WARN_CFLAGS} ${DBG_CFLAGS}