bpo-38809: Windows build scripts use python.exe from virtual envs (GH-17164)

https://bugs.python.org/issue38809 (cherry picked from commit ee703cb) Co-authored-by: Tal Einat <taleinat+github@gmail.com>

Author: miss-islington

Misc/NEWS.d/next/Build/2019-11-15-09-25-44.bpo-38809.9jwta6.rst

@@ -0,0 +1,2 @@
+On Windows, build scripts will now recognize and use python.exe from an
+active virtual env.

PCbuild/find_python.bat

@@ -24,6 +24,9 @@
 :begin_search
 @set PYTHON=
 
+@rem If there is an active virtual env, use that one
+@if NOT "%VIRTUAL_ENV%"=="" (set PYTHON="%VIRTUAL_ENV%\Scripts\python.exe") & (set _Py_Python_Source=found in virtual env) & goto :found
+
 @set _Py_EXTERNALS_DIR=%EXTERNALS_DIR%
 @if "%_Py_EXTERNALS_DIR%"=="" (set _Py_EXTERNALS_DIR=%~dp0\..\externals)