Enable icecc to find compilers outside PATH when building locally

Teach find_compiler and friends to conditionally use the full compiler
path name. This enables users to have (cross-)compilers outside PATH,
and is especially useful in combination with ccache and CCACHE_PREFIX.
In this case no directory of symbolic links is needed nor does the
(cross-)compiler have to be in the PATH.

Accomplish this by saving away the full compiler name and use it in
cases when the compiler name looks like an absolute or relative path.

Signed-off-by: David Vest <davve@opera.com>