diff --git a/GNUmakefile b/GNUmakefile
index fc71ae6..4009f40 100644
--- a/GNUmakefile
+++ b/GNUmakefile
@@ -257,13 +257,13 @@ checkdeps:
 	    exit 1;                                                \
 	  fi;                                                      \
 	done
-	@for i in $(CHECKDEPS_HDRS); do                                 \
-	  if echo "#include <$${i}>" | $(CC) -E - >/dev/null 2>&1; then \
-	    printf "checking for $${i}... \033[1;32myes\033[0m\n";      \
-	  else                                                          \
-	    printf "checking for $${i}... \033[1;31mno\033[0m\n";       \
-	    exit 1;                                                     \
-	  fi;                                                           \
+	@for i in $(CHECKDEPS_HDRS); do                                                                                               \
+	  if echo "#include <$${i}>" | $(CC) -isystem /usr/local/$(HDR_DIR) -isystem /usr/X11R6/$(HDR_DIR) -E - >/dev/null 2>&1; then \
+	    printf "checking for $${i}... \033[1;32myes\033[0m\n";                                                                    \
+	  else                                                                                                                        \
+	    printf "checking for $${i}... \033[1;31mno\033[0m\n";                                                                     \
+	    exit 1;                                                                                                                   \
+	  fi;                                                                                                                         \
 	done
 	@for i in $(CHECKDEPS_TYPES); do                                                                                      \
 	  if echo "#include <stdint.h>\n#include <stddef.h>\n$${i} x;" | $(CC) -x c -S - -o /dev/stdout >/dev/null 2>&1; then \