Check compiler support for -fno-default-inline and -pipe

clang was spewing warnings about the unrecognized -fno-default-inline.  (Oddly,
it warns only with -c, not when compiling directly to an executable.)  For
completeness we also check -pipe, even though clang is OK with that one.

It should be fine to omit either flag.  gcc -fno-default-inline drops the
implicit 'inline' annotation on functions defined inside a class scope, but
'inline' is only a hint anyway.  -fno-default-inline does not change linkage.
-pipe is merely a compile speed optimization.

configure.ac

@@ -101,6 +101,17 @@ AS_IF([test x"$hardening" != x"no"], [
 AC_SUBST([HARDEN_CFLAGS])
 AC_SUBST([HARDEN_LDFLAGS])
 
+# Also check for a few non-hardening-related flags.
+MISC_CXXFLAGS=""
+AC_LANG_PUSH(C++)
+AX_CHECK_COMPILE_FLAG([-fno-default-inline],
+  [MISC_CXXFLAGS="$MISC_CXXFLAGS -fno-default-inline"], [], [-Werror])
+AX_CHECK_COMPILE_FLAG([-pipe],
+  [MISC_CXXFLAGS="$MISC_CXXFLAGS -pipe"], [], [-Werror])
+AC_LANG_POP(C++)
+AC_SUBST([MISC_CXXFLAGS])
+
+# End of flag tests.
 CC="$saved_CC"
 CXX="$saved_CXX"
 LD="$saved_LD"