build: more robustly check for fcf-protection support

When using Clang 7, we may end up trying to use the flag when it won't
work properly, which can lead to confusing errors. i.e:
```bash
/usr/bin/ld: error: ... <corrupt x86 feature size: 0x8>
```

Use CHECK_LINK_FLAG & --fatal-warnings to ensure we wont use the flag in this case.

configure.ac

@@ -819,7 +819,10 @@ if test x$use_hardening != xno; then
   AX_CHECK_COMPILE_FLAG([-Wstack-protector],[HARDENED_CXXFLAGS="$HARDENED_CXXFLAGS -Wstack-protector"])
   AX_CHECK_COMPILE_FLAG([-fstack-protector-all],[HARDENED_CXXFLAGS="$HARDENED_CXXFLAGS -fstack-protector-all"])
 
-  AX_CHECK_COMPILE_FLAG([-fcf-protection=full],[HARDENED_CXXFLAGS="$HARDENED_CXXFLAGS -fcf-protection=full"])
+  dnl -fcf-protection used with Clang 7 causes ld to emit warnings:
+  dnl ld: error: ... <corrupt x86 feature size: 0x8>
+  dnl Use CHECK_LINK_FLAG & --fatal-warnings to ensure we wont use the flag in this case.
+  AX_CHECK_LINK_FLAG([-fcf-protection=full],[HARDENED_CXXFLAGS="$HARDENED_CXXFLAGS -fcf-protection=full"],, [[$LDFLAG_WERROR]])
 
   dnl stack-clash-protection does not work properly when building for Windows.
   dnl We use the test case from https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90458