Moved Sphinx detection outside of the PYTHONINTERP_FOUND block.

Who knows, maybe the user's Sphinx installation uses a Python we couldn't find.

cmake/OpenCVDetectPython.cmake

@@ -74,8 +74,9 @@ if(PYTHONINTERP_FOUND)
                       OUTPUT_STRIP_TRAILING_WHITESPACE)
     endif()
   endif(NOT ANDROID AND NOT IOS)
+endif()
 
-  if(BUILD_DOCS)
+if(BUILD_DOCS)
   find_host_program(SPHINX_BUILD sphinx-build)
   if(SPHINX_BUILD)
       execute_process(COMMAND "${SPHINX_BUILD}"
@@ -88,5 +89,4 @@ if(PYTHONINTERP_FOUND)
         message(STATUS "Found Sphinx ${SPHINX_VERSION}: ${SPHINX_BUILD}")
       endif()
   endif()
-  endif(BUILD_DOCS)
-endif()
+endif(BUILD_DOCS)