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

@@ -150,26 +150,41 @@ foreach(_CURRENT_VERSION ${_Python_VERSIONS})
     PATH_SUFFIXES python${_CURRENT_VERSION}/config
   )
 
-  set(PYTHON_FRAMEWORK_INCLUDES)
-  if(Python_FRAMEWORKS AND NOT PYTHON_INCLUDE_DIR)
-    foreach(dir ${Python_FRAMEWORKS})
-      list(APPEND PYTHON_FRAMEWORK_INCLUDES
-           ${dir}/Versions/${_CURRENT_VERSION}/include)
-    endforeach()
-  endif()
+  # 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()
 
-  find_path(PYTHON_INCLUDE_DIR
-    NAMES Python.h
-    PATHS
-      ${PYTHON_FRAMEWORK_INCLUDES}
-      [HKEY_LOCAL_MACHINE\\SOFTWARE\\Python\\PythonCore\\${_CURRENT_VERSION}\\InstallPath]/include
-      [HKEY_CURRENT_USER\\SOFTWARE\\Python\\PythonCore\\${_CURRENT_VERSION}\\InstallPath]/include
-    PATH_SUFFIXES
-      python${_CURRENT_VERSION}mu
-      python${_CURRENT_VERSION}m
-      python${_CURRENT_VERSION}u
-      python${_CURRENT_VERSION}
-  )
+    # Add framework directories to the search paths
+    set(PYTHON_FRAMEWORK_INCLUDES)
+    if(Python_FRAMEWORKS AND NOT PYTHON_INCLUDE_DIR)
+      foreach(dir ${Python_FRAMEWORKS})
+        list(APPEND PYTHON_FRAMEWORK_INCLUDES
+          ${dir}/Versions/${_CURRENT_VERSION}/include)
+      endforeach()
+    endif()
+
+    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
+        [HKEY_CURRENT_USER\\SOFTWARE\\Python\\PythonCore\\${_CURRENT_VERSION}\\InstallPath]/include
+      PATH_SUFFIXES
+        python${_CURRENT_VERSION}mu
+        python${_CURRENT_VERSION}m
+        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