updated macos installer for unit tests

ci_scripts/install_class_osx.sh

@@ -6,10 +6,14 @@ rm -rf class_public
 git clone --depth=1000 https://github.com/lesgourg/class_public.git
 cd class_public
 
-sed -i.bak -e 's/^CC/#CC/g' Makefile
-sed -i.bak -e 's/^OPTFLAG =/OPTFLAG = ${CFLAGS} ${LDFLAGS}/g' Makefile
-sed -i.bak -e 's/^#CCFLAG +=/CCFLAG +=/g' Makefile
-sed -i.bak -e 's/^#CCFLAG =/CCFLAG =/g' Makefile
+# if you are running on macos 11.6 and higher, uncomment the following line
+sed -i.bak -e 's/gcc/gcc-9/g' Makefile
+
+# if you are running on macos 10.15 and lower, uncomment the following 4 lines
+#sed -i.bak -e 's/^CC/#CC/g' Makefile
+#sed -i.bak -e 's/^OPTFLAG =/OPTFLAG = ${CFLAGS} ${LDFLAGS}/g' Makefile
+#sed -i.bak -e 's/^#CCFLAG +=/CCFLAG +=/g' Makefile
+#sed -i.bak -e 's/^#CCFLAG =/CCFLAG =/g' Makefile
 
 make -j4