fix build OS when setting macos

Author: krichprollsch

build-tools/utils.sh

@@ -25,11 +25,12 @@ esac
 
 : "${OS:=unset}"
 case "$OS" in
+  macos)  OS="mac" ;;
   unset)
     case "$OSTYPE" in
       darwin*)  OS="mac" ;;
       linux*)   OS="linux" ;;
       *)        fail "unsupported platform: ${OSTYPE}"
     esac
     ;;
-esac
+esac