depcomp: improve comments about the 'gcc' depmode It is not only needed by obsolescent gcc compilers (pre-3.x), but also by modern compiler like IBM C/C++. State that ... * lib/depcomp: ... here... * gen-testsuite-part: ... and here, where we generate the 'depcomp*.tap' tests. Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>