Hardcoding CFLAGS without checking if the flags are supported isn't a
good idea. For instance, if no CFLAGS where given, it would
unconditionally add -O4 as the optimization flag. Clang doesn't have
-O4. It understands it, but will issue a warning for every compilation
unit that -O4 is equal to -O3. Clang also doesn't have
-fexpensive-optimizations.

This changes it to actually check for supported compiler flags and only
adds them if the compiler understands them.

There are several details here.

CFLAGS where previously set at the start, before AC_PROG_CC since that
adds default flags. That check now instead seta a EMPTY_CFLAGS marker
that's later used to determine if we should add the previously hardcoded
flags.

The variable supported_cflags will be filled with the flags the compiler
supports and eventually CFLAGS will be set to supported_cflags.

For correct detection, -Werror (or equivalent) needs to be in effect for
the compiler to issue an error for unknown flags. This is therefore
checked first and added to CFLAGS while performing the checks. This is not
added to supported_cflags and does therefore not end up on the final
CFLAGS.

Next, the check for -fPIC has been migrated from a platform check to a
compiler flag check. If found to work, it's added both to supported_cflags
and to LDFLAGS as it needs to be present during linking as well.

Then comes the check for the flags that were previously added
unconditionally if CFLAGS was empty, in the order they were before.
Of particular note here is the split in logic to check for the
optimization flag. It will check -O4 first. If -O4 is supported, it will
not check for others. If -O4 is not supported, it will check -O3.

The final compiler flags check is for disabling warnings. This is new and,
like -fPIC, is done regardless of whether CFLAGS was empty or not. The
only check here so far is for -Wno-language-extension-token which Clang
needs to stop complaining about inline assembly statements. Note that the
checks for warnings needs sed to convert from the -Wno-<warning> form to
the -W<warning> form, so a AC_PROG_SED check is added to the top with the
other program checks. This is because the -Wno-<warning> form isn't
consulted unless there is warning (at least by GCC) so the flag would
always be accepted. The -W<warning> form mean the compiler is forced to
consider it up front.