package/brltty: fix cross-compilation

package/brltty: fix cross-compilation

brltty has a very inventive buildsystem, where it internall runs
./configure for the build machine In doing so, it generates a list
of make variables to define what the build machine supports, like
it does for the target.

However, the build variables are generated with a convoluted sed
script that scans the target list, and appends _FOR_BUILD to each
target variables. Then, both lists are included from the Makefile,
on the assumption that the build variables will not clash with the
target variables.

Where it gets interesting, is that that sed script considers the
variables names to match '[A-Za-z][A-Za-z0-9_]*'

And there we see why ATSPI2_PACKAGE does not match: it contains a
digit.

So, some build variables will inevitably override target ones.

Fix that by simply expanding the matching regexp to allow digits
in variable names.

Fixes:
    http://autobuild.buildroot.org/results/a37/a37782b3cfc1a96cc129db8fade20a36a7b2d470/
    http://autobuild.buildroot.org/results/97e/97edc6a47d2140968e84b409cdc960604e5896f2/

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Mario Lang <mlang@blind.guru>
Cc: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>