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

perl/Makefile

@@ -4,11 +4,4 @@ GLOBAL_CXXFLAGS += -g -Wall
 
 -include Makefile.config
 
-OPTIMIZE = 1
-
-ifeq ($(OPTIMIZE), 1)
-  GLOBAL_CFLAGS += -O3
-  GLOBAL_CXXFLAGS += -O3
-endif
-
 include mk/lib.mk