Re: [freetype2] master 576403f 1/2: [builds] Relocate `FTMODULE_H'.

[Werner LEMBERG]

[A preliminary remark: We have to improve FreeType's terminology.  For
 example, in `ftconfig.h` there is some talk about 'build directory'.
 Users might assume that this refers to the directory where
 compilation puts its object files.  However, this is not the case.  I
 will probably do `s/BUILD_DIR/PLATFORM_DIR/` for clarity and update
 the documentation accordingly.]

> We host custom/generated ftoption.h and ftconfig.h in either
> BUILD_DIR or DEVEL_DIR.

This is not correct.  In a normal build with srcdir != builddir and
srcdir read-only, we either have the unmodified `ftoption.h` and
`ftconfig.h` files (the latter is only of interest if none of the
platforms in the `builds` directory are sufficient, which is rather
unlikely today) in `SRC_DIR`, or we have user-modified files in
`OBJ_DIR`.

A user-defined `BUILD_DIR` comes only into play if you need to support
exotic operating systems not covered with the stuff in the `builds`
directory.

`DEVEL_DIR` is just a swap-in to quickly replace `ftoption.h` for
development builds; it is set by `make devel` (this target exists only
for pure GNU make builds; meson and cmake don't support it).

In summary, we have the following order for include files.

  $(DEVEL_DIR)         (set by `make devel`, defaulting to being
                       undefined) 
  $(BUILD_DIR)         (set by the various `detect.mk` files for a
                       pure GNU make build, hard-coded otherwise)
  $(TOP_DIR)/include

You can find exactly this in `freetype.mk`.

`ftoption.h` is handled separately by FTOPTION_H, which in turn is
configured by pure GNU make builds and hard-coded otherwise.

> Only one of them is always defined but not both.  Why should
> ftmodule.h be different?

They are not different.  If a user wants to restrict the modules, the
file `modules.cfg` should be adjusted and put into `OBJ_DIR`.  A
corresponding `ftmodule.h` file will then be automatically created by
make (and meson, but not cmake AFAICS).

> This should work but slightly later in toplevel.mk.
> 
> ifdef DEVEL_BUILD
>   FTMODULE_H ?= $(DEVEL_DIR)/ftmodule.h
> else
>   FTMODULE_H ?=$(BUILD_DIR)/ftmodule.h
> endif

Such an approach is not necessary, I think.

> We do not

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    Werner