re-enable the case-sensitive check and add notes for macos user

README.md

@@ -26,6 +26,8 @@ For MacOS, you need to install some GNU tools which the openwrt is required:
 $ brew install gnu-tar gnu-getopt findutils m4
 ```
 
+> Linux source tree requires a case-sensitive file system, make sure your workspace is APFS (case-sensitive) before you start.
+
 The openwrt [check gnu-getopt version](./openwrt/include/prereq-build.mk) via the `gnugetopt` command name, so the following link is also required:
 
 ```sh

openwrt/include/prereq-build.mk

@@ -20,10 +20,10 @@ $(eval $(call TestHostCommand,working-make, \
 	$(MAKE) -v | grep -E 'Make (3\.8[1-9]|3\.9[0-9]|[4-9]\.)'))
 
 # disabled because macos doesn't own this.c
-# $(eval $(call TestHostCommand,case-sensitive-fs, \
-# 	OpenWrt can only be built on a case-sensitive filesystem, \
-# 	rm -f $(TMP_DIR)/test.*; touch $(TMP_DIR)/test.fs; \
-# 		test ! -f $(TMP_DIR)/test.FS))
+$(eval $(call TestHostCommand,case-sensitive-fs, \
+	OpenWrt can only be built on a case-sensitive filesystem, \
+	rm -f $(TMP_DIR)/test.*; touch $(TMP_DIR)/test.fs; \
+		test ! -f $(TMP_DIR)/test.FS))
 
 $(eval $(call SetupHostCommand,gcc, \
 	Please install the GNU C Compiler (gcc), \