configure.ac : Don't zap '-g' from $CFLAGS.

The '-g' was in-correctly getting zapped from all CFLAGS expressions
including eg:

    -mfloat-gprs=double (for powerpc e500) -> -mfloatprs=double.

Reported-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
configure.ac : Relax automake version requirements.

Suggested-by: Jan Stary <hans@stare.cz>
Fix some minor word duplication

Signed-off-by: Erik de Castro Lopo <erikd@mega-nerd.com>