Add verification on gcc version and command.

Most importantly, if version is specified and command is not, check for
g++-$version and failing that, check if g++ -dumpversion returns the
version we've asked about.

Patch from Moritz Hassert.
Addresses #4667.


[SVN r65633]

tools/build

@@ -1 +1 @@
-Subproject commit 302866c325533365c465310d6217f63fb1394a83
+Subproject commit 78aa937eccf2f622e82953333b08905cc67f4a25