Redo the Sphinx check to do the same thing on all platforms.

This also fixes the bug where the Sphinx version gets detected as
"Sphinx vX.Y.Z" instead of "X.Y.Z" on Unix-like systems.

cmake/OpenCVDetectPython.cmake

@@ -104,18 +104,12 @@ if(PYTHON_EXECUTABLE)
   if(BUILD_DOCS)
     find_host_program(SPHINX_BUILD sphinx-build)
     if(SPHINX_BUILD)
-        if(UNIX)
-            execute_process(COMMAND sh -c "${SPHINX_BUILD} -_ 2>&1 | sed -ne 1p"
-                             RESULT_VARIABLE SPHINX_PROCESS
-                             OUTPUT_VARIABLE SPHINX_VERSION
-                             OUTPUT_STRIP_TRAILING_WHITESPACE)
-        else()
-            execute_process(COMMAND ${PYTHON_EXECUTABLE} -c "import sphinx; print sphinx.__version__"
-                            RESULT_VARIABLE SPHINX_PROCESS
-                            OUTPUT_VARIABLE SPHINX_VERSION
-                            OUTPUT_STRIP_TRAILING_WHITESPACE)
-        endif()
-        if(SPHINX_PROCESS EQUAL 0)
+        execute_process(COMMAND "${SPHINX_BUILD}"
+                        OUTPUT_QUIET
+                        ERROR_VARIABLE SPHINX_OUTPUT
+                        OUTPUT_STRIP_TRAILING_WHITESPACE)
+        if(SPHINX_OUTPUT MATCHES "^Sphinx v([0-9][^ \n]*)")
+          set(SPHINX_VERSION "${CMAKE_MATCH_1}")
           set(HAVE_SPHINX 1)
           message(STATUS "Found Sphinx ${SPHINX_VERSION}: ${SPHINX_BUILD}")
         endif()