diff --git a/mkspecs/common/ghs-base.conf b/mkspecs/common/ghs-base.conf
index eba9680349..3fd4cf70a7 100644
--- a/mkspecs/common/ghs-base.conf
+++ b/mkspecs/common/ghs-base.conf
@@ -29,6 +29,7 @@ QMAKE_CXXFLAGS_STATIC_LIB += $$QMAKE_CFLAGS_STATIC_LIB
 QMAKE_CXXFLAGS_APP     += $$QMAKE_CFLAGS_APP
 QMAKE_CXXFLAGS_YACC    += $$QMAKE_CFLAGS_YACC
 QMAKE_CXXFLAGS_HIDESYMS += $$QMAKE_CFLAGS_HIDESYMS
+QMAKE_CFLAGS_C99        = -std=c99
 QMAKE_CXXFLAGS_CXX11   += --c++11 --thread_local_storage
 QMAKE_CXXFLAGS_EXCEPTIONS_ON += --exceptions