add support for building out-of-sources builds

[slyfox]

I like run ./autogen.sh and and ./configure scripts from locations other, than source tree.
It is very convinient to run builds for various architectures/configurations against one actual source.

usage example:
$ cd /tmp
$ git clone path-to-mc
$ chmod -R u-w mc/ # freeze sources (just for tests)

# let's craft 'mc-build'
# (and mc-build-{arm, x86_64, i386, ppc, mingw32}-{slang, charset, glib{1,2}} in real world)
# and build sources there:

$ mkdir mc-build
$ cd mc-build
$ ../mc/autogen.sh
$ ./configure --prefix=/tmp/mc-test/
$ make
$ make install

or at least to generate ./configure in /tmp/mc tree, but to be able t run it from other locations to have source tree untouched