configure: Add workaround for ccache and clang

Test if ccache is interfering with semantic analysis of macros,
disable its habit of trying to compile already pre-processed
versions of code if so. ccache attempts to save time by compiling
pre-processed versions of code, but this disturbs clang's static
analysis enough to produce false positives.

ccache allows us to disable this feature, opting instead to
compile the original version instead of its preprocessed version.
This makes ccache much slower for cache misses, but at least it
becomes usable with QEMU/clang.

This workaround only activates for users using ccache AND clang,
and only if their configuration is observed to be producing warnings.
You may need to clear your ccache for builds started without -Werror,
as those may continue to produce warnings from the cache.

Thanks to Peter Eisentraut for his writeup on the issue:
http://peter.eisentraut.org/blog/2014/12/01/ccache-and-clang-part-3/Signed-off-by: NJohn Snow <jsnow@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Message-id: 1427324259-1481-5-git-send-email-jsnow@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

configure

@@ -103,7 +103,8 @@ update_cxxflags() {
 }
 
 compile_object() {
-  do_cc $QEMU_CFLAGS -c -o $TMPO $TMPC
+  local_cflags="$1"
+  do_cc $QEMU_CFLAGS $local_cflags -c -o $TMPO $TMPC
 }
 
 compile_prog() {
@@ -4209,6 +4210,33 @@ if compile_prog "" "" ; then
     getauxval=yes
 fi
 
+########################################
+# check if ccache is interfering with
+# semantic analysis of macros
+
+ccache_cpp2=no
+cat > $TMPC << EOF
+static const int Z = 1;
+#define fn() ({ Z; })
+#define TAUT(X) ((X) == Z)
+#define PAREN(X, Y) (X == Y)
+#define ID(X) (X)
+int main(int argc, char *argv[])
+{
+    int x = 0, y = 0;
+    x = ID(x);
+    x = fn();
+    fn();
+    if (PAREN(x, y)) return 0;
+    if (TAUT(Z)) return 0;
+    return 0;
+}
+EOF
+
+if ! compile_object "-Werror"; then
+    ccache_cpp2=yes
+fi
+
 ##########################################
 # End of CC checks
 # After here, no more $cc or $ld runs
@@ -5502,6 +5530,10 @@ if test "$numa" = "yes"; then
   echo "CONFIG_NUMA=y" >> $config_host_mak
 fi
 
+if test "$ccache_cpp2" = "yes"; then
+  echo "export CCACHE_CPP2=y" >> $config_host_mak
+fi
+
 # build tree in object directory in case the source is not in the current directory
 DIRS="tests tests/tcg tests/tcg/cris tests/tcg/lm32 tests/libqos tests/qapi-schema tests/tcg/xtensa tests/qemu-iotests"
 DIRS="$DIRS fsdev"