depcomp: style changes to Portland Group Compilers support

depcomp: style changes to Portland Group Compilers support

* lib/depcomp (pgcc): Quote 'like this', not `like this'.  Other minor
quoting improvements.  Remove a commented-out command.  In comments,
use proper capitalization and punctuation.  Make a more consistent use
of whitespace.  Make fatal error messages more nicely formatted, and
send them to standard error rather than to standard output.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>