Don't hardcode the -O2 compiler flag

autoconf authors apparently decided that setting `-O2` by default was a good idea. I disagree, and Nix has its own way of deciding that (with `OPTIMIZE={0,1}`). Explicitly set `CFLAGS` and `CXXFLAGS` in the configure script to disable that behaviour.

Fix #9965

configure.ac

@@ -47,6 +47,10 @@ AC_DEFINE_UNQUOTED(SYSTEM, ["$system"], [platform identifier ('cpu-os')])
 # State should be stored in /nix/var, unless the user overrides it explicitly.
 test "$localstatedir" = '${prefix}/var' && localstatedir=/nix/var
 
+# Assign a default value to C{,XX}FLAGS as the default configure script sets them
+# to -O2 otherwise, which we don't want to have hardcoded
+CFLAGS=${CFLAGS-""}
+CXXFLAGS=${CXXFLAGS-""}
 
 AC_PROG_CC
 AC_PROG_CXX