configure.ac: Remove -fstack-protector-strong from LLVM flags.

-fstack-protector-strong is not supported by clang.

This patch fixes this build error on Fedora 20 with clang.

  CXX      gallivm/lp_bld_debug.lo
clang: error: unknown argument: '-fstack-protector-strong'

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=75010
Signed-off-by: Vinson Lee <vlee@freedesktop.org>

configure.ac

@@ -1591,7 +1591,8 @@ strip_unwanted_llvm_flags() {
 	-e 's/-fno-exceptions\>//g' \
 	-e 's/-fomit-frame-pointer\>//g' \
 	-e 's/-fvisibility-inlines-hidden\>//g' \
-	-e 's/-fPIC\>//g'
+	-e 's/-fPIC\>//g' \
+	-e 's/-fstack-protector-strong\>//g'
 }