Revert "Use $CC and $CPP if provided"

This reverts commit 8f9c4a78a4.

Makefile

@@ -1,5 +1,5 @@
-CC ?= gcc
-CPP ?= g++
+CC = gcc
+CPP = g++
 AR = ar
 LD = ld
 

build-support/factor.sh

@@ -97,7 +97,7 @@ set_md5sum() {
 set_gcc() {
     case $OS in
         openbsd) ensure_program_installed egcc; CC=egcc;;
-        *) CC=${CC:=gcc};;
+        *) CC=gcc;;
     esac
 }