infra/pkg-meson: allow packages to pass custom compiler/linker flags

infra/pkg-meson: allow packages to pass custom compiler/linker flags

Meson does not allow to pass CFLAGS/LDFLAGS/CXXFLAGS via the environment
or via command-line arguments or options (instead, those flags from the
environment are passed to the host compiler, which is seldom what we
need). The only way to pas those flags is via the cross-compilation.conf
file.

Add LIBFOO_CFLAGS, LIBFOO_LDFLAGS and LIBFOO_CXXFLAGS variables to allow
packages to provide their own flags, possibly overriding the generic
ones entirely, as we allow for other infras. Those per-package flags will
then be used to generate the per-package cross-compilation.conf.

This means that the meson infra is the first and only infra for which
FOO_CFLAGS, FOO_LDFLAGS, and FOO_CXXFLAGS are meaningful, while for the
other infras, they are just variables private to the package itself.
Instead of naming those variables after the meson infra (e.g.
FOO_MESON_CFLAGS), we name them with a generic name, as maybe, just
maybe, we could also change the other infras to also recognise those
variables.

Just like for the HOST_MESON_SED_CFLAGS etc., we need to add auxiliary
variables to do convert the shell-formatted argument list into the
JSON-formatted list that meson expects. We can't use a pure-make
construct because the CFLAGS can contain quoting that needs to be
expanded by the shell. Similarly, we need a condition on the strip'ed
variable to avoid passing empty arguments.

To mimic this feature for packages that are built from the SDK, we also
install a templatised version of cross-compilation.conf, with three new
placeholders for custom flags. If a user wants to build a package that
needs custom flags, they can use that template to generate a per-package
cross-compilation.conf.

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Adam Duskett <aduskett@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Tested-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>