build: Use -isystem instead of -I
diff --git a/Makefile b/Makefile
index 936d703..aa3acb5 100644
--- a/Makefile
+++ b/Makefile
@@ -128,7 +128,7 @@ CFLAGS ?= -g -Wall -Wextra -Wno-unused-variable
LDFLAGS ?=
# Internal makefile variables
-MK_CPPFLAGS := -Iinclude/bsd/ \
+MK_CPPFLAGS := -isystem include/bsd/ \
-DLIBBSD_OVERLAY -DLIBBSD_DISABLE_DEPRECATED \
-D_GNU_SOURCE -D__REENTRANT
MK_CFLAGS :=