illumos gate

Today, we use the environment variable GCC_ROOT to indicate where to find the root of a GCC installation. While testing on support for multiple shadows, I discovered that the GCC_ROOT environment variable causes gcc to change the compiler search path that it uses to find where it executes from. For example, take a look at the following:

$ /home/rm/src/gcc7/proto.strap/usr/gcc/7/bin/gcc -c tmp.c
$ export GCC_ROOT=/home/rm/src/gcc7/proto.strap/usr/gcc/4
$ /home/rm/src/gcc7/proto.strap/usr/gcc/7/bin/gcc -c tmp.c
gcc: error trying to exec 'cc1': execvp: No such file or directory

When I look at the specs, the major thing that changes is the internal
search path in the presence. Here's the search path without GCC_ROOT set:

COMPILER_PATH=/home/rm/src/gcc7/proto.strap/usr/gcc/7/bin/../libexec/gcc/i386-pc-solaris2.11/7.3.0/:/home/rm/src/gcc7/proto.strap/usr/gcc/7/bin/../libexec/gcc/:/usr/ccs/bin/

But with it set:

COMPILER_PATH=/home/rm/src/gcc7/proto.strap/usr/gcc/4/libexec/gcc/:/usr/ccs/bin/

Based on this, and that the gcc source code admits that it uses GCC_ROOT as a special value, we probably shouldn't use it in our Makefiles.