builds/freetype.mk: Invoke mkdocs as a Python module

FreeType's `refdoc` target currently allows users to override the
default Python path, which is useful for testing and development.

In contrast, `mkdocs` is invoked via the default Python path.

Invoking `mkdocs` via Python's module syntax allows for greater
flexibility, although there is no change for the default use case.

builds/freetype.mk

@@ -299,7 +299,7 @@ refdoc:
                   $(PUBLIC_DIR)/config/*.h \
                   $(PUBLIC_DIR)/cache/*.h
 	@echo Building static site...
-	cd $(DOC_DIR) && mkdocs build
+	cd $(DOC_DIR) && $(PYTHON) -m mkdocs build
 	@echo Done.
 
 # Variables for running `refdoc' with Python's `virtualenv'.  The