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++