Should reject relative paths to executables.

[Paul Hargrove]

Currently we silently accept relatives paths for at least CC, CXX.
I suspect (but have not checked) that --with-python and --with-gmake do too.
Part of the problem comes from our trust in type -p, which I've discovered is happy with valid relative paths:

$ type -p ../../../../../usr/bin/g++
../../../../../usr/bin/g++

All of these executables are eventually run from somewhere other than from the original build directory, and things break.

We have the option to "absify" or reject such inputs.
As the current title of this issue conveys, my plan is to reject relative paths.
This can (and thus should) done in a single shell function that is also responsible for absification of bare names (like CXX=icpc) and verification that full-paths name existing executables.

[Paul Hargrove]

proposed solution in pull request 197

[Paul Hargrove]

configure: update absification of CC and CXX

This commit updates the logic which absifies CC and CXX to use the new check_tool_path() function. Relative to the prior logic, this adds rejection of relative paths and slightly improves error messages.

Resolves issue 350.

→ <<cset d6ce99a6bd6c>>