build: fix windows smp build with gyp

Gyp will try a parallel build if it detect the system has >1 processor.
This functionality depends on the Python "multiprocessing" package. The
multiprocessing package on Windows requires that the top-level module is
importable as a module, see:

  http://docs.python.org/2/library/multiprocessing.html#windows

This fixes issue #984.

This is a back-port of commit 2445467 from the master branch.

README.md

@@ -91,12 +91,12 @@ Or:
 
 Unix users run
 
-    ./gyp_uv -f make
+    ./gyp_uv.py -f make
     make -C out
 
 Macintosh users run
 
-    ./gyp_uv -f xcode
+    ./gyp_uv.py -f xcode
     xcodebuild -project uv.xcodeproj -configuration Release -target All
 
 Note for UNIX users: compile your project with `-D_LARGEFILE_SOURCE` and

vcbuild.bat

@@ -91,7 +91,7 @@ exit /b 1
 
 :have_gyp
 if not defined PYTHON set PYTHON="python"
-%PYTHON% gyp_uv -Dtarget_arch=%target_arch% -Dlibrary=%library%
+%PYTHON% gyp_uv.py -Dtarget_arch=%target_arch% -Dlibrary=%library%
 if errorlevel 1 goto create-msvs-files-failed
 if not exist uv.sln goto create-msvs-files-failed
 echo Project files generated.