configure: simplify bigendian check

The preferred way to use LTO is --enable-lto but often times packagers
still end up with -flto in cflags for various reasons. Using grep
on binary object files is brittle and relies on specific object
representation, which in the case of LLVM bitcode, debug information or
other intermediary formats can fail silently.

This patch changes the check to a more commonly used define for GCC
style compilers. More checks may be needed to cover other potential
compilers that don't use the __BYTE_ORDER__ define.

Signed-off-by: J. Dekker <jdek@itanimul.li>

configure

@@ -6120,11 +6120,7 @@ extern_prefix=${sym%%ff_extern*}
 
 check_cc pragma_deprecated "" '_Pragma("GCC diagnostic push") _Pragma("GCC diagnostic ignored \"-Wdeprecated-declarations\"")'
 
-# The global variable ensures the bits appear unchanged in the object file.
-test_cc <<EOF || die "endian test failed"
-unsigned int endian = 'B' << 24 | 'I' << 16 | 'G' << 8 | 'E';
-EOF
-od -t x1 $TMPO | grep -q '42 *49 *47 *45' && enable bigendian
+test_cpp_condition stdlib.h "defined(__BYTE_ORDER__) && (__BYTE_ORDER__ == __ORDER_BIG_ENDIAN__)" && enable bigendian
 
 check_cc const_nan math.h "struct { double d; } static const bar[] = { { NAN } }"