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>

1 files changed, 5 insertions, 1 deletions

diff --git a/lib/depcomp b/lib/depcomp
index ee84bf286..693fe0496 100755
--- a/lib/depcomp
+++ b/lib/depcomp
@@ -142,13 +142,17 @@ gcc3)
   ;;
 
 gcc)
+## Note that this doesn't just cater to obsosete pre-3.x GCC compilers.
+## but also to in-use compilers like IMB xlc/xlC and the HP C compiler.
+## (see the conditional assignment to $gccflag above).
 ## There are various ways to get dependency output from gcc.  Here's
 ## why we pick this rather obscure method:
 ## - Don't want to use -MD because we'd like the dependencies to end
 ##   up in a subdir.  Having to rename by hand is ugly.
 ##   (We might end up doing this anyway to support other compilers.)
 ## - The DEPENDENCIES_OUTPUT environment variable makes gcc act like
-##   -MM, not -M (despite what the docs say).
+##   -MM, not -M (despite what the docs say).  Also, it might not be
+##   supported by the other compilers which use the 'gcc' depmode.
 ## - Using -M directly means running the compiler twice (even worse
 ##   than renaming).
   if test -z "$gccflag"; then