diff --git a/src/configure.ac b/src/configure.ac
index 918c36c..2c24ae4 100644
--- a/src/configure.ac
+++ b/src/configure.ac
@@ -1005,9 +1005,11 @@
       AC_SUBST(vi_cv_perl_xsubpp)
       dnl Remove "-fno-something", it breaks using cproto.
       dnl Remove "-fdebug-prefix-map", it isn't supported by clang.
+      dnl Remove "FORTIFY_SOURCE", it will be defined twice.
       perlcppflags=`$vi_cv_path_perl -Mlib=$srcdir -MExtUtils::Embed \
-	      -e 'ccflags;perl_inc;print"\n"' | sed -e 's/-fno[[^ ]]*//' \
-			-e 's/-fdebug-prefix-map[[^ ]]*//g'`
+		-e 'ccflags;perl_inc;print"\n"' | sed -e 's/-fno[[^ ]]*//' \
+		-e 's/-fdebug-prefix-map[[^ ]]*//g' \
+		-e 's/\(-Wp,\)\?-D_FORTIFY_SOURCE=.//g`
       dnl Remove "-lc", it breaks on FreeBSD when using "-pthread".
       perllibs=`cd $srcdir; $vi_cv_path_perl -MExtUtils::Embed -e 'ldopts' | \
 		sed -e '/Warning/d' -e '/Note (probably harmless)/d' \