build failure on solaris 2.7

[Rupert Wood]

I wrote:

> Certainly you can work around this by setting CONFIG_SHELL to bash.

Oops; need BUILD_SHELL as well.

I've dug out my old test-case; ltconfig was aborting on the line

  . $cache_file

But when I added an echo after to check if it was getting that far:

  . $cache_file
  echo "Returned $?"

everything worked fine (!). I experimented a bit further and found that (.
$cache_file) works too, whereupon I accepted the workaround, assumed it was
fairly me-specific and put it aside for later. Odd shell behaviour is beyond
my expertise.

Rup.