build: generate man-page only when appropriate

* configure.ac: detect compilation under git (vs tarball), and
cross-compilation.
* Makefile.am:
1) re-generate (and clean) man-page only when building from git.
2) if building from git AND cross-compiling, don't add target for
   man-page - there's no way to generate it.

Possible future improvement: If cross-compliing from git, AND user have
'binfmt' configured to run non-native binaries, then the man-page can be
generated.

Reference:
http://lists.gnu.org/archive/html/bug-hello/2014-07/msg00001.html
build: revert AM_TESTS_ENVIRONMENT to TESTS_ENVIRONMENT

* Makefile.am: prefer TESTS_ENVIRONMENT rather than AM_TESTS_ENVIRONMENT
  to avoid problems with older automake versions.  For example Travis-CI
  is not working because of this issue, or Debian stable cannot be used
  to build upstream GNU Hello.
* configure.ac: automake 1.12 or newer will not allow combination of
  parallel-tests and TESTS_ENVIRONMENT.

Reference:
http://lists.gnu.org/archive/html/bug-hello/2014-07/msg00003.html