Honor user's CFLAGS

[noloader]

The project's required CFLAGS are gathered in UTH_CFLAGS and used in the target recipes. It leaves CFLAGS for the user. Also see https://www.gnu.org/prep/standards/html_node/Command-Variables.html.

[Quuxplusone]

This seems reasonable; I like the guidelines in https://www.gnu.org/prep/standards/html_node/Command-Variables.html . However:

• this now needs rebasing (thanks for reminding me to merge the no-more-murmurhash patch!)
• if we're putting the user's CFLAGS at the end of the line, then should we put the user's CPPFLAGS (preprocessor flags) at the end of the line as well?
• s/UTH_CFLAGS/HASH_CFLAGS/ — let's keep the number of prefixes we use to a minimum

[Quuxplusone]

Besides my high-level comments in the "Conversation" tab: should this now use ${CFLAGS} instead of ${EXTRA_CFLAGS}? I believe the purpose of EXTRA_CFLAGS was so that the user can pass their own extra stuff, but if we're going to respect their CFLAGS then they don't need this extra mechanism.