This is more reliable than using `which`, which doesn't work if the CXX
environment variable is set to something else than an executable (e.g
contains options), and is apparently less portable on some systems.

See:
 * http://lists.geany.org/pipermail/devel/2009-September/001367.html,
   which lead to 5bb28825.
 * https://sourceforge.net/p/geany/bugs/455/, which lead to a revert
   of the above, 5b9605a9.

Fixes #829.