Rework the implementation of not including -Werror for specific files,
to a way that does not require hardcoding modified automake output back
into its input.
* Makefile.am (AM_CXXFLAGS, BASECXXFLAGS): Seperate out -Werror.
(autoload.o): Drop this rule, -finline-functions is no longer required
and -Werror is acceptable here now we have __attribute__ ((used)).
(WARNONLY_CFLAGS, inilex.o): Drop these.
(EXTRA_PROGRAMS): Add a fake "warnonly" program.
(warnonly_CXXFLAGS): Set to the $(BASECXXFLAGS).
(warnonly_SOURCES): Set to "inilex.cc".
(EXTRA_setup_SOURCES): Set to $(warnonly_SOURCES).
(setup_SOURCES): Remove "inilex.cc".
(setup_LDADD): Add "warnonly-inilex.o".