configure: disable FORTIFY_SOURCE under clang

Some versions of clang may have difficulty compiling glibc headers when
-D_FORTIFY_SOURCE is used. For example, Clang++ 3.5.0-9.fc22 cannot
compile glibc's stdio headers when -D_FORTIFY_SOURCE=2 is used. This
manifests currently as build failures with clang and any arm target.

According to LLVM dev Richard Smith, clang does not target or support
FORTIFY_SOURCE + glibc, and it should not be relied on.
"It's still an unsupported combination, and while it might compile, some
of the checks are unlikely to work because they require a frontend
inliner to be useful"

See: http://lists.llvm.org/pipermail/cfe-dev/2015-November/045846.html

Conclusion: disable fortify-source if we appear to be using clang instead
of testing for compile success or failure, which may be incidental or not
indicative of proper support of the feature.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <1446583422-10153-1-git-send-email-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -264,6 +264,7 @@ rdma=""
 gprof="no"
 debug_tcg="no"
 debug="no"
+fortify_source=""
 strip_opt="yes"
 tcg_interpreter="no"
 bigendian="no"
@@ -879,6 +880,7 @@ for opt do
       debug_tcg="yes"
       debug="yes"
       strip_opt="no"
+      fortify_source="no"
   ;;
   --enable-sparse) sparse="yes"
   ;;
@@ -4439,6 +4441,19 @@ if ! compile_object "-Werror"; then
     ccache_cpp2=yes
 fi
 
+#################################################
+# clang does not support glibc + FORTIFY_SOURCE.
+
+if test "$fortify_source" != "no"; then
+  if echo | $cc -dM -E - | grep __clang__ > /dev/null 2>&1 ; then
+    fortify_source="no";
+  elif echo | $cxx -dM -E - | grep __clang__ > /dev/null 2>&1 ; then
+    fortify_source="no";
+  else
+    fortify_source="yes"
+  fi
+fi
+
 ##########################################
 # End of CC checks
 # After here, no more $cc or $ld runs
@@ -4446,7 +4461,7 @@ fi
 if test "$gcov" = "yes" ; then
   CFLAGS="-fprofile-arcs -ftest-coverage -g $CFLAGS"
   LDFLAGS="-fprofile-arcs -ftest-coverage $LDFLAGS"
-elif test "$debug" = "no" ; then
+elif test "$fortify_source" = "yes" ; then
   CFLAGS="-O2 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 $CFLAGS"
 fi