RFR: 8044733: (xs) common/autoconf/configure script doesn't properly detect missing tools

[Mike Duigou]

Hello all;

This is a small changeset to fix a problem where on some platforms (Solaris and possibly others) the configure script doesn't properly detect the absence of autoconf or mercurial because which returns a string for the failure condition.

The recipe used was previously in hgforest.sh for it's detection of the mercurial executable.

issue: https://bugs.openjdk.java.net/browse/JDK-8044733
webrev: http://cr.openjdk.java.net/~mduigou/JDK-8044733/0/webrev

Mike