Add tests for scons.tigris.org issue#1723.

Ensure CCVERSION/CXXVERSION detected actually is from CC/CXX used.
Respect preset CC/CXX values detecting cc/c++/gcc/g++ Tools.

As the user-preset values for CC/CXX should rule always, also respect them in
exists() and generate() methods of these Tools. As a result, the value for
CCVERSION/CXXVERSION does match the CC/CXX compiler used (issue#1723).
Delegate linker Tool.exists() to CC/CXX Tool.exists().

Even for linking, need to respect CC/CXX specified by the user (issue#1723).
And when CC is specified but not CXX, assume the user knows there is nothing
to link with CXX, and delegate to CC Tool.exists() only. However, this somehow
should be synchronized with link.smart_link() to choose the correct linker.
The _r in AIX xlc_r means reentrant, not relocatable.

It does not make any sense to use 'xlc' for CC and 'xlc_r' for SHCC, as the
'_r' does stand for 'reentrant' rather than 'relocatable' or similar.

Avoid 'egrep' to parse the lslpp output, it's easy enough within python.

Needs output streams of _subproc.dummyPopen to be iterable.
- adding test for issue #2311, which appears to be fixed already
Merged pull request #138; Respect preset CC/CXX values.

This prevents SCons from overwriting user's selected CC/CXX compiler
in the cc/c++/gcc/gcc tools.  Fixes issue #1723.
Closed branch issue1723
Closed branch issue1723
Merged in haubi/scons, pull request #139:
Delegate linker Tool.exists() to CC/CXX Tool.exists().
See issue #1723.
Closing branch
Closed branch linker-exists-when-compiler-exists
Merged in haubi/scons/aix-xlc_r-is-for-threads (pull request #140)

The _r in AIX xlc_r means reentrant, not relocatable.
Merged in dirkbaechle/scons (pull request #141)

Test for issue #2311