Allow custom compiler via environment variable for src/cc

The build system supports using CC to control the C compiler
and CXX to control the c++ compiler used but src/cc was hardcoded
to use the binary "gcc". Now it respects the custom value of
$CC environment variable if it exists.

src/cc/Makefile_custom

@@ -1,5 +1,4 @@
 SHELL		= /bin/sh
-CC		= gcc
 CC_DARWIN	= g++-8
 CC_WIN		= x86_64-w64-mingw32-gcc-posix
 CC_AARCH64	= aarch64-linux-gnu-g++

util/build.sh

@@ -137,7 +137,7 @@ CONFIG_SITE="$PWD/depends/$HOST/share/config.site" ./configure "$HARDENING_ARG"
 WD=$PWD
 cd src/cc
 echo $PWD
-./makecustom
+CC=${CC:-gcc} ./makecustom
 cd $WD
 
 # Build RandomX