Don't clobber toolchain defaults.

(TODO: Fix upstream's GCC_FORTIFY_SOURCE macro or just replace it with the
autoconf-archive one (AX_ADD_FORTIFY_SOURCE)).

https://bugs.gentoo.org/895522
--- a/configure.ac
+++ b/configure.ac
@@ -37,7 +37,6 @@ if test -n "$GCC"; then
    CFLAGS="$CFLAGS -Wall -Wshadow -Wwrite-strings"
    GCC_WEXTRA
    GCC_STACK_PROTECT_CC
-   GCC_FORTIFY_SOURCE
    GCC_FORMAT_SECURITY
 dnl Uncomment the line below to compile with additional warnings enabled.
 dnl CFLAGS="$CFLAGS -pedantic -Wpointer-arith -Wcast-qual -Wcast-align -Wstrict-prototypes -Wmissing-prototypes -Wmissing-declarations -Wnested-externs"
@@ -49,6 +48,8 @@ else
 fi
 AC_PROG_INSTALL
 
+AX_ADD_FORTIFY_SOURCE
+
 dnl Checks for libraries.
 dnl Solaris 10 needs -lnsl for gethostbyname() and -lsocket for socket().
 dnl Linux, {Free,Open,Net,Dragonfly}BSD and MacOS X do not.