Issue #25136: Support Apple Xcode 7's new textual SDK stub libraries.

As of Xcode 7, SDKs for Apple platforms now include textual-format stub
libraries whose file names have a .tbd extension rather than the
standard OS X .dylib extension.  The Apple compiler tool chain handles
these stub libraries transparently and the installed system shared libraries
are still .dylibs.  However, the new stub libraries cause problems for
third-party programs that support building with Apple SDKs and make
build-time decisions based on the presence or paths of system-supplied
shared libraries in the SDK.  In particular, building Python itself with
an SDK fails to find system-supplied libraries during setup.py's build of
standard library extension modules.  The solution is to have
find_library_file() in Distutils search for .tbd files, along with
the existing types (.a, .so, and .dylib).  Patch by Tim Smith.

setup.py

@@ -136,6 +136,22 @@ def find_library_file(compiler, libname, std_dirs, paths):
         p = p.rstrip(os.sep)
 
         if host_platform == 'darwin' and is_macosx_sdk_path(p):
+            # Note that, as of Xcode 7, Apple SDKs may contain textual stub
+            # libraries with .tbd extensions rather than the normal .dylib
+            # shared libraries installed in /.  The Apple compiler tool
+            # chain handles this transparently but it can cause problems
+            # for programs that are being built with an SDK and searching
+            # for specific libraries.  Distutils find_library_file() now
+            # knows to also search for and return .tbd files.  But callers
+            # of find_library_file need to keep in mind that the base filename
+            # of the returned SDK library file might have a different extension
+            # from that of the library file installed on the running system,
+            # for example:
+            #   /Applications/Xcode.app/Contents/Developer/Platforms/
+            #       MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/
+            #       usr/lib/libedit.tbd
+            # vs
+            #   /usr/lib/libedit.dylib
             if os.path.join(sysroot, p[1:]) == dirname:
                 return [ ]