My configure.ac contains
AC_PROG_CXX
and my Makefile.am contains
AM_CXXFLAGS= -Werror -O3
and the resulting 'configure' script keeps on adding -O2 to the mix in addition to O3.
I have this suspicion that I'm supposed to do something in configure.ac to declare my preference for O3, but I can't find it in the doc. How do I tell it that I want to control the optimization level?
No, you are not supposed to add anything to configure.ac to indicate a preference for -O3. That sort of thing belongs in a user's CONFIG_SITE, not in configure.ac
However, if you insist on breaking the convention, you can do something like:
: ${CXXFLAGS=-Werror -O3}
Note that this line must appear before AC_PROG_CXX (or any macro that AC_REQUIRES it, such as LT_INIT, as that will cause AC_PROG_CXX to be emitted into the configure script before this line.)
The prior answer is correct in that it should be the installer's choice what flags are used. If you're still wondering where the -g -O2 comes from (likely), it is described at http://www.gnu.org/s/hello/manual/autoconf/C_002b_002b-Compiler.html.
That page does suggest "If your package does not like this default, then it is acceptable to insert the line ‘: ${CXXFLAGS=""}’ after AC_INIT and before AC_PROG_CXX to select an empty default instead."
