--- libnumbertext-1.0.2/configure.ac
+++ libnumbertext-1.0.2/configure.ac
@@ -22,7 +22,7 @@
 
 dnl Checks for typedefs, structures, and compiler characteristics.
 AC_LANG([C++])
-CXXFLAGS='-Wall -std=c++11'
+CXXFLAGS="$CXXFLAGS -Wall -std=c++11"
 
 AC_CHECK_HEADERS([codecvt regex])