autoconf: Allow overriding CFLAGS/CXXFLAGS from outside.

As is normal for autoconf-based projects.

For example, it is a common use case to do

    ./configure CXXFLAGS=-O0

This did not work for nix until now, because the `CXXFLAGS=` declaration
would unconditionally erase what the user had specified.

The custom `OPTIMIZE` flag is removed, but the default `-O3` is retained;
autoconf would default to `-g -O2` by default otherwise as documented on:

https://www.gnu.org/software/autoconf/manual/autoconf-2.60/html_node/C-Compiler.html
https://www.gnu.org/software/autoconf/manual/autoconf-2.60/html_node/C_002b_002b-Compiler.html

0 files changed, 0 insertions, 0 deletions