<rdar://problem/7745082> Make it possible to build WebKit for older Mac OS X versions from the current Mac OS X version

Reviewed by Tim Hatcher.

Introduce TARGET_MAC_OS_X_VERSION_MAJOR to represent the Mac OS X version that is being targeted.  It defaults to the
current Mac OS X version unless otherwise specified.

Key off TARGET_MAC_OS_X_VERSION_MAJOR where we'd previously been keying off MAC_OS_X_VERSION_MAJOR.

Explicitly map from the target Mac OS X version to the preferred compiler since Xcode's default compiler choice
may not be usable when targetting a different Mac OS X version.

Key off TARGET_GCC_VERSION rather than MAC_OS_X_VERSION_MAJOR in locations where we'd previously been keying off
MAC_OS_X_VERSION_MAJOR but the decision is really related to the compiler version being used.

JavaScriptCore:

* Configurations/Base.xcconfig:
* Configurations/DebugRelease.xcconfig:
* Configurations/FeatureDefines.xcconfig:
* Configurations/JavaScriptCore.xcconfig:
* Configurations/Version.xcconfig:

JavaScriptGlue:

* Configurations/Base.xcconfig:
* Configurations/DebugRelease.xcconfig:
* Configurations/Version.xcconfig:
* JavaScriptGlue.xcodeproj/project.pbxproj:

WebCore:

* Configurations/Base.xcconfig:
* Configurations/DebugRelease.xcconfig:
* Configurations/FeatureDefines.xcconfig:
* Configurations/Version.xcconfig:
* Configurations/WebCore.xcconfig:

WebKit/mac:

* Configurations/Base.xcconfig:
* Configurations/DebugRelease.xcconfig:
* Configurations/FeatureDefines.xcconfig:
* Configurations/Version.xcconfig:

WebKitTools:

* DumpRenderTree/mac/Configurations/Base.xcconfig:
* DumpRenderTree/mac/Configurations/DebugRelease.xcconfig:
* DumpRenderTree/mac/DumpRenderTree.mm: Wrap the include of mach-o/getsect.h in 'extern "C"' as some versions of the
header in older SDKs do not do this inside the header.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@55867 268f45cc-cd09-0410-ab3c-d52691b4dbfc