build-sys: Check behavior of -Werror=shadow before deciding to use it

gcc versions 4.6 and earlier used to complain when a local variable
shadows a global function, 4.7 and above only complain if a local
variable shadows a global variable.

Fix this by checking whether gcc 4.7+ behavior is in place before
deciding to use -Werror=shadow in $(CFLAGS), by using a custom test
program source that shadows a global function with a local variable and
confirming that -Werror=shadow does not make the compile to break.

Tested:
- On gcc 4.7 and 4.8, confirmed nothing changed (other than the order of
  the -Werror=shadow argument, going to the end of CFLAGS.)
- On gcc 4.6, confirmed by looking at the config.log output that the
  check for -Werror=shadow failed and it was not included in CFLAGS.
- Ran `make V=1` to confirm -Werror=shadow was still in use, introduced
  a bogus shadowing issue and confirmed it was caught when building with
  a recent gcc.

1 files changed, 11 insertions, 1 deletions

diff --git a/configure.ac b/configure.ac
index d71cb07366..5cdd1d1462 100644
--- a/configure.ac
+++ b/configure.ac
@@ -171,7 +171,6 @@ CC_CHECK_FLAGS_APPEND([with_cflags], [CFLAGS], [\
         -Werror=implicit-function-declaration \
         -Werror=missing-declarations \
         -Werror=return-type \
-        -Werror=shadow \
         -Wstrict-prototypes \
         -Wredundant-decls \
         -Wmissing-noreturn \
@@ -196,6 +195,17 @@ CC_CHECK_FLAGS_APPEND([with_cflags], [CFLAGS], [\
         -fPIE \
         --param=ssp-buffer-size=4])
 
+CC_CHECK_FLAG_APPEND([with_cflags], [CFLAGS], [-Werror=shadow], [
+#include <time.h>
+#include <inttypes.h>
+typedef uint64_t usec_t;
+usec_t now(clockid_t clock);
+int main(void) {
+        struct timespec now;
+        return 0;
+}
+])
+
 AS_CASE([$CC], [*clang*],
         [CC_CHECK_FLAGS_APPEND([with_cppflags], [CPPFLAGS], [\
                -Wno-typedef-redefinition \