Set python exe to simplify venv dir

GDAL uses the python exe to determine what directory structure to use.
When using the system python it defaults to local/lib/... but when it
detects a venv it uses lib/..

This matches the other python libraries so its a small improvement.

Author: MJohnson459

SuperBuild/cmake/External-GDAL.cmake

@@ -17,6 +17,7 @@ ExternalProject_Add(${_proj_name}
     -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
     -DCMAKE_INSTALL_PREFIX:PATH=${SB_INSTALL_DIR}
     -DGDAL_PYTHON_INSTALL_PREFIX=${PYTHON_HOME}
+    -DPython_EXECUTABLE=${PYTHON_EXE_PATH}
     -DBUILD_PYTHON_BINDINGS=ON
     ${WIN32_CMAKE_ARGS}
   #--Build step-----------------