Regenerate the loader source whenever make is called

The following variables can be customised when compiling the library:

~~~make
GL_NAMESPACE        ?= gl
GL_API              ?= gl
GL_PROFILE          ?= core
GL_VERSION          ?= 4.3
GL_EXTENSIONS       ?=
GL_FULL             ?=
GL_XML              ?= $(DEPS_DIR)/khronos-api/$(GL_API).xml
~~~

For example:

~~~
GL_VERSION=2.1 GL_PROFILE=compatability make lib
~~~
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