Fix Python-related redefinitions for builds using mingw-w64

[sav-ix]

Fixes #140.
Tested for builds using mingw-w64 with <Debug|Release><Shared|Static> configurations.
UPDATE: But it broke MSVC builds. Any suggestions?

[stefanseefeld]

By "broke MSVC builds", are you referring to the Appveyor build failure ? That's an unrelated issue with the SCons build logic. Boost.Build is not involved in the CI builds. If not, can you show an error log ? (I'm only a casual Windows user, so I may not be able to help.)

[sav-ix]

Travis failures forced me to test this patch for MSVC builds. Which I missed to do before publishing this PR.
For builds using MSVC with Debug+Static and Release+Shared configurations got the same errors:

error: "gcc" is not a known value of feature <toolset>
error: legal values: "msvc"

(see logs in attachment).

[stefanseefeld]

I don't think this is doing what you expect. As far as I understand Boost.Build syntax, the last line above defines as requireents:

• toolset=gcc
• target-os=windows
• cxxflags=-DMS_WIN64, whenever link=shared

It looks like what you intend to do is to define -DMS_WIN64 whenever all three features (toolset, target-os, and link) have the given values, yes ?
Please ask on the boost or boost.build mailing list what the right syntax for this is.
(I'm also not sure how this would fix the `"gcc is not a known value..." error. That's another thing to ask on boost.build ML).
Sorry I can't help more with this. Boost.Build is complicated and badly documented, which is one reason why I'm trying to move away from it...

[sav-ix]

I was trying to add rules to Boost.Python build system:
IF build using mingw-w64 THEN add -D_hypot=hypot to compiler keys,
IF build using mingw-w64 AND layout shared THEN add -DMS_WIN64 to compiler keys,
based on other Jamfiles as samples. But Boost.Build was more complicated then expected.

Please ask on the boost or boost.build mailing list what the right syntax for this is.

Or we can invite Boost.Build Developers to join this discussion.
@raffienficiaud, could you give a tip, how to solve this issue correctly?