bpo-36245: Avoid problems when building in a directory containing spaces. (GH-12241)

(cherry picked from commit 7ee88bf3e59493137a775368165c5c5fe1ed7f46)

Co-authored-by: Jess <jess@gmail.com>
diff --git a/PCbuild/get_externals.bat b/PCbuild/get_externals.bat
index 175a051..cbeb5a1 100644
--- a/PCbuild/get_externals.bat
+++ b/PCbuild/get_externals.bat
@@ -41,7 +41,7 @@
 if "%ORG%"=="" (set ORG=python)
 call "%PCBUILD%\find_python.bat" "%PYTHON%"
 
-if "%PYTHON%"=="" (
+if NOT DEFINED PYTHON (
     where /Q git || echo Python 3.6 could not be found or installed, and git.exe is not on your PATH && exit /B 1
 )
 
@@ -60,7 +60,7 @@
 for %%e in (%libraries%) do (
     if exist "%EXTERNALS_DIR%\%%e" (
         echo.%%e already exists, skipping.
-    ) else if "%PYTHON%"=="" (
+    ) else if NOT DEFINED PYTHON (
         echo.Fetching %%e with git...
         git clone --depth 1 https://github.com/%ORG%/cpython-source-deps --branch %%e "%EXTERNALS_DIR%\%%e"
     ) else (
@@ -79,7 +79,7 @@
 for %%b in (%binaries%) do (
     if exist "%EXTERNALS_DIR%\%%b" (
         echo.%%b already exists, skipping.
-    ) else if "%PYTHON%"=="" (
+    ) else if NOT DEFINED PYTHON (
         echo.Fetching %%b with git...
         git clone --depth 1 https://github.com/%ORG%/cpython-bin-deps --branch %%b "%EXTERNALS_DIR%\%%b"
     ) else (