build: Don't hardcode $(LIBTOOL)
When building freetype-demos with slibtool (https://dev.midipix.org/cross/slibtool) it ignores the $(LIBTOOL)
variable in a few places.
export MAKEFLAGS='LIBTOOL=rdlibtool'
The build should be consistent about respecting the variable so that either GNU libtool or slibtool can be used.