Fix test_cmake_build failure with bare python exe name (fix #783)

Besides appearing in the CMake GUI, the `:FILENAME` specifier changes
behavior as well:

cmake -DPYTHON_EXECUTABLE=python ..  # FAIL, can't find python
cmake -DPYTHON_EXECUTABLE=/path/to/python ..  # OK
cmake -DPYTHON_EXECUTABLE:FILENAME=python ..  # OK
cmake -DPYTHON_EXECUTABLE:FILENAME=/path/to/python ..  # OK

tests/CMakeLists.txt

@@ -197,7 +197,7 @@ if(NOT CMAKE_VERSION VERSION_LESS 3.1)
 
     set(build_options "-DCMAKE_PREFIX_PATH=${PROJECT_BINARY_DIR}/mock_install"
                       "-DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}"
-                      "-DPYTHON_EXECUTABLE=${PYTHON_EXECUTABLE}"
+                      "-DPYTHON_EXECUTABLE:FILEPATH=${PYTHON_EXECUTABLE}"
                       "-DPYBIND11_CPP_STANDARD=${PYBIND11_CPP_STANDARD}")
     if(NOT ARG_INSTALL)
       list(APPEND build_options "-DPYBIND11_PROJECT_DIR=${PROJECT_SOURCE_DIR}")