Revert "autoconf: Allow overriding CFLAGS/CXXFLAGS from outside."

This reverts commit 717e821b99. It's
much more convenient to do 'make OPTIMIZE=0'.

perl/Makefile

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

perl/configure.ac

@@ -2,10 +2,8 @@ AC_INIT(nix-perl, m4_esyscmd([bash -c "echo -n $(cat ../.version)$VERSION_SUFFIX
 AC_CONFIG_SRCDIR(MANIFEST)
 AC_CONFIG_AUX_DIR(../config)
 
-# Set default flags for nix (as per AC_PROG_CC/CXX docs),
-# while still allowing the user to override them from the command line.
-: ${CFLAGS="-O3"}
-: ${CXXFLAGS="-O3"}
+CFLAGS=
+CXXFLAGS=
 AC_PROG_CC
 AC_PROG_CXX
 AX_CXX_COMPILE_STDCXX_11