configure: Split valgrind test into pragma test and valgrind.h test

Split the configure test that checks for valgrind into two, one
part checking whether we have the gcc pragma to disable unused-but-set
variables, and the other part checking for the existence of valgrind.h.
The first of these has to be compiled with -Werror and the second
does not and shouldn't generate any warnings.

This (a) allows us to enable "make errors in configure tests be
build failures" and (b) enables use of valgrind on systems with
a gcc which doesn't know about -Wunused-but-set-varibale, like
Debian squeeze.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

configure

@@ -2872,15 +2872,30 @@ if compile_prog "" "" ; then
     linux_magic_h=yes
 fi
 
+########################################
+# check whether we can disable the -Wunused-but-set-variable
+# option with a pragma (this is needed to silence a warning in
+# some versions of the valgrind VALGRIND_STACK_DEREGISTER macro.)
+# This test has to be compiled with -Werror as otherwise an
+# unknown pragma is only a warning.
+pragma_disable_unused_but_set=no
+cat > $TMPC << EOF
+#pragma GCC diagnostic ignored "-Wunused-but-set-variable"
+int main(void) {
+    return 0;
+}
+EOF
+if compile_prog "-Werror" "" ; then
+    pragma_disable_unused_but_set=yes
+fi
+
 ########################################
 # check if we have valgrind/valgrind.h
 
 valgrind_h=no
 cat > $TMPC << EOF
 #include <valgrind/valgrind.h>
-#pragma GCC diagnostic ignored "-Wunused-but-set-variable"
 int main(void) {
-  VALGRIND_STACK_DEREGISTER(0);
   return 0;
 }
 EOF
@@ -3397,6 +3412,10 @@ if test "$linux_magic_h" = "yes" ; then
   echo "CONFIG_LINUX_MAGIC_H=y" >> $config_host_mak
 fi
 
+if test "$pragma_disable_unused_but_set" = "yes" ; then
+  echo "CONFIG_PRAGMA_DISABLE_UNUSED_BUT_SET=y" >> $config_host_mak
+fi
+
 if test "$valgrind_h" = "yes" ; then
   echo "CONFIG_VALGRIND_H=y" >> $config_host_mak
 fi

coroutine-ucontext.c

@@ -200,14 +200,18 @@ Coroutine *qemu_coroutine_new(void)
 }
 
 #ifdef CONFIG_VALGRIND_H
+#ifdef CONFIG_PRAGMA_DISABLE_UNUSED_BUT_SET
 /* Work around an unused variable in the valgrind.h macro... */
 #pragma GCC diagnostic ignored "-Wunused-but-set-variable"
+#endif
 static inline void valgrind_stack_deregister(CoroutineUContext *co)
 {
     VALGRIND_STACK_DEREGISTER(co->valgrind_stack_id);
 }
+#ifdef CONFIG_PRAGMA_DISABLE_UNUSED_BUT_SET
 #pragma GCC diagnostic error "-Wunused-but-set-variable"
 #endif
+#endif
 
 void qemu_coroutine_delete(Coroutine *co_)
 {