updated.

config/depcomp

@@ -74,6 +74,9 @@ tmpdepfile=${tmpdepfile-`echo "$depfile" | sed 's/\.\([^.]*\)$/.T\1/'`}
 
 rm -f "$tmpdepfile"
 
+# Avoid interferences from the environment.
+gccflag= dashmflag=
+
 # Some modes work just like other modes, but use different flags.  We
 # parameterize here, but still list the modes in the big case below,
 # to make depend.m4 easier to write.  Note that we *cannot* use a case
@@ -108,7 +111,7 @@ if test "$depmode" = msvc7msys; then
 fi
 
 if test "$depmode" = xlc; then
-   # IBM C/C++ Compilers xlc/xlC can output gcc-like dependency informations.
+   # IBM C/C++ Compilers xlc/xlC can output gcc-like dependency information.
    gccflag=-qmakedep=gcc,-MF
    depmode=gcc
 fi
@@ -142,13 +145,17 @@ 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