Use $CC and $CPP if provided

Let the user select the compiler he wants to use to compile Factor
by using the supplied $CC and $CPP instead of just discarding them.
If not present, we default to gcc and g++ (the current behaviour).

Makefile

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

build-support/factor.sh

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