build: downgrade keyword-macro from an error to a warning with clang

refs #3235

Change-Id: If687229c721d41d7a06b9014c1d9130b798bca64
diff --git a/.waf-tools/default-compiler-flags.py b/.waf-tools/default-compiler-flags.py
index 35907b9..f82fe10 100644
--- a/.waf-tools/default-compiler-flags.py
+++ b/.waf-tools/default-compiler-flags.py
@@ -161,6 +161,7 @@
                               '-Wno-unused-local-typedef', # Bugs #2657 and #3209
                               '-Wno-error=unneeded-internal-declaration', # Bug #1588
                               '-Wno-error=deprecated-register',
+                              '-Wno-error=keyword-macro', # Bug #3235
                               ]
         return flags