Fixed bug with nested groups in Xcode 5 ide generator.
Merge pull request #220 from mattrudder/xcode5-generator-fix

Fixed bug with nested groups in Xcode 5 IDE generator.
Fix a couple of memory leaks @ shutdown.
Use clang on Linux.
Add valgrind instrumentation to linear allocator.

Mark allocated memory as unintialized.
Mark all memory as inaccessible to begin with.
Mark rolled back memory as inaccessible.
On Linux checked builds, enable valgrind API calls.
On checked builds, use malloc so we get heap checking w/ valgrind as well.
Rework Source resolution to support nested SourceDir

Closes GH-75.
Expose option to continue on error

Closes GH-214
Add /EHsc to silence warning.

Closes GH-219
Separate compile and link time PDB files.

Closes GH-82
Rework threading abstractions to run on Windows XP.
Actually add /EHsc to options.

It helps if you use the right option name.
Update README with WinXP note.
Add MSVC6 support.
Xcode5 generator: Avoid derefing nil hints
Xcode5 generator: Avoid using nil Resources
Xcode5 generator: Misc fixes

Fix indentation
Fix no sources emitted without hints.
Xcode5: Clean up indentation further
Xcode5 generator: Clean up table iteration
Allow running other tools than pkg-config.

Useful for e.g. wx-config.
Clean up option handling to ignore newlines and other junk
Add support for MSVC IDE variant mappings that include subvariants

For example, in your MSVC generation hints, you can now write:
VariantMappings = {
  ['release-default']    = 'Release'
  ['release-analyze']    = 'Release-Analyze',
}
...and you'll get a Release and Release-Analyze configuration in your IDE.

One could already achieve this with the FullMapping hint, but that forces
one to enumerate all build configurations, which is tedious when the
number of configurations is high.