package/meson: fix empty arguments in cross-compilation.conf

package/meson: fix empty arguments in cross-compilation.conf

When TARGET_CFLAGS (or _LDFLAGS or _CXXFLAGS) are empty, but were
constructed by appending other variables, like:

    TARGET_CFLAGS = $(SOMETHING) $(SOMETHING_ELSE)

and both variables are empty, then $(TARGET_CFLAGS) is _not_ the
null-string; it's value is a string made of a single space.

This means that the construct:

    $(if $(TARGET_CFLAGS),true,false)

will in fact return 'true'.

In our case, it means that we will call:

    `printf '"%s", ' `

which expands to just:

    "",

which we are then happy to insert as-is in the generated
cross-compilation.conf.

Then meson, will happily call the compiler with an empty argument.

The compiler is less happy, though:

    arm-none-linux-gnueabi-gcc: error: : No such file or directory

And this is not even trivial to debug either... The only clue being that
there seems to be something missing between ': :'

We fix that testing the $(strip)ed value. We can still pass the
non-$(strip) expansion, because the shell will just do it for us, and we
are then sure there is at least one non-blank word in there.

Thanks a lot to Adam for his invaluable help debugging this!

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Adam Duskett <aduskett@gmail.com>
Cc: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>