diff --git a/configure b/configure
index 2616af58..e9131851 100755
--- a/configure
+++ b/configure
@@ -2405,7 +2405,7 @@ case $host_os in *\ *) host_os=`echo "$host_os" | sed 's/ /-/g'`;; esac
 
 
 
-CXXFLAGS="-D_GNU_SOURCE"
+CXXFLAGS="$CXXFLAGS -D_GNU_SOURCE"
 INCLUDES=""
 LIBS=""
 DEPEND=""
diff --git a/configure.in b/configure.in
index fd8ccbee..a72a0067 100644
--- a/configure.in
+++ b/configure.in
@@ -3,7 +3,7 @@ AC_INIT([znc], [0.051])
 AC_PROG_CXX
 AC_CANONICAL_HOST
 
-CXXFLAGS="-D_GNU_SOURCE"
+CXXFLAGS="$CXXFLAGS -D_GNU_SOURCE"
 INCLUDES=""
 LIBS=""
 DEPEND=""