GCC warning option tweaks

- WTH are we compiling with -Wno-char-subscripts? This appears to have
  been there since the dawn of times, and at least nowadays removing
  it causes no new warnings.
- Re-enable -Wstrict-prototypes, either NSS headers have gotten fixed
  or GCC has gotten smarter and no longer whines about broken
  system headers.

configure.ac

@@ -35,9 +35,7 @@ fi
 AS=${AS-as}
 AC_SUBST(AS)
 if test "$GCC" = yes; then
-    CFLAGS="$CFLAGS -fPIC -DPIC -D_REENTRANT -Wall -Wpointer-arith -Wmissing-prototypes -Wno-char-subscripts"
-    # XXX disabled for now due to noise from NSPR headers
-    # CFLAGS="$CFLAGS -Wstrict-prototypes"
+    CFLAGS="$CFLAGS -fPIC -DPIC -D_REENTRANT -Wall -Wpointer-arith -Wmissing-prototypes -Wstrict-prototypes"
     cflags_to_try="-fno-strict-aliasing -fstack-protector -Wempty-body"
     AC_MSG_CHECKING([supported compiler flags])
     old_cflags=$CFLAGS