FindPythonLibs: Match include dir to library version

This commit ensures that FindPythonLibs has found the library before
before the search for the include dir begins.  The library prefix and
version can then be used to find the matching include dir.

Modules/FindPythonLibs.cmake

@@ -150,6 +150,18 @@ foreach(_CURRENT_VERSION ${_Python_VERSIONS})
     PATH_SUFFIXES python${_CURRENT_VERSION}/config
   )
 
+  # Don't search for include dir until library location is known
+  if(PYTHON_LIBRARY)
+
+    # Use the library's install prefix as a hint
+    set(_Python_INCLUDE_PATH_HINT)
+    get_filename_component(_PREFIX ${PYTHON_LIBRARY} PATH)
+    get_filename_component(_PREFIX ${_PREFIX} PATH)
+    if(_PREFIX)
+      set(_Python_INCLUDE_PATH_HINT ${_PREFIX}/include)
+    endif()
+
+    # Add framework directories to the search paths
     set(PYTHON_FRAMEWORK_INCLUDES)
     if(Python_FRAMEWORKS AND NOT PYTHON_INCLUDE_DIR)
       foreach(dir ${Python_FRAMEWORKS})
@@ -160,6 +172,8 @@ foreach(_CURRENT_VERSION ${_Python_VERSIONS})
 
     find_path(PYTHON_INCLUDE_DIR
       NAMES Python.h
+      HINTS
+        ${_Python_INCLUDE_PATH_HINT}
       PATHS
         ${PYTHON_FRAMEWORK_INCLUDES}
         [HKEY_LOCAL_MACHINE\\SOFTWARE\\Python\\PythonCore\\${_CURRENT_VERSION}\\InstallPath]/include
@@ -170,6 +184,7 @@ foreach(_CURRENT_VERSION ${_Python_VERSIONS})
         python${_CURRENT_VERSION}u
         python${_CURRENT_VERSION}
     )
+  endif()
 
   # For backward compatibility, set PYTHON_INCLUDE_PATH.
   set(PYTHON_INCLUDE_PATH "${PYTHON_INCLUDE_DIR}")
@@ -187,6 +202,8 @@ foreach(_CURRENT_VERSION ${_Python_VERSIONS})
   endif()
 endforeach()
 
+unset(_Python_INCLUDE_PATH_HINT)
+
 mark_as_advanced(
   PYTHON_DEBUG_LIBRARY
   PYTHON_LIBRARY