package/pkg-meson: support per-package directories

Currently, package/meson/meson.mk generates a single global
cross-compilation.conf file, with the path to the compiler, cflags,
ldflags, and various other details. This file is then used when
building all meson-based packages.

This causes two problems:

 - It is not compatible with per-package directories, because with
   per-package folders, we need to use a different compiler, and
   possibly CFLAGS/LDFLAGS for each package.

 - It is not possible to define per package CFLAGS. Indeed, when
   cross-compiling, meson doesn't support passing CFLAGS through the
   environment, only the CFLAGS from cross-compilation.conf are taken
   into account.

For this reason, this commit:

 - Introduces a per-package cross-compilation.conf, which is generated
   by the pkg-meson infrastructure in the "configure" step right
   before calling meson. The file is generated in $(@D)/build/, and
   because it is generated within a given package "configure" step,
   the compiler path is the one of this package.

 - Keeps the global cross-compilation.conf in $(HOST_DIR)/etc/meson/,
   for the SDK use-case of Buildroot. Since we want the final and
   global values of the compiler path, CFLAGS and LDFLAGS, generating
   this global cross-compilation.conf is moved to a
   TARGET_FINALIZE_HOOKS. If we were keeping this as a
   HOST_MESON_POST_INSTALL_HOOKS, it would contain values specific to
   the host-meson package.

For now, we don't yet support per-package CFLAGS/LDFLAGS, but having
such per-package cross-compilation.conf is a necessary preparation to
achieve this goal.

This commit has been tested by building all Buildroot packages that
use meson: json-glib, systemd, enlightenment, at-spi2-core, ncmpc,
libmpdclient and ncmpc.

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
[Thomas:
 - add extended commit log
 - in pkg-meson.mk, re-use variables defined in meson.mk to do the
   replacement of CFLAGS/LDFLAGS/CXXFLAGS
 - move the generation of the global cross-compilation.conf to a
   TARGET_FINALIZE_HOOKS
 - testing with per-package folders]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Tested-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>