[BRLTTY] cross-compilation of brltty & pkg-config

[Samuel Thibault]

Hello,

Among the warnings raised by Debian tools, there is:

W: autotools-pkg-config-macro-not-cross-compilation-safe
N:
N:   The package appears to use AC_PATH_PROG to discover the location of
N:   pkg-config(1). This macro fails to select the correct version to
N:   support cross-compilation.
N:
N:   A better way would be to use the PKG_PROG_PKG_CONFIG macro from pkg.m4
N:   and then using the $PKG_CONFIG shell variable.
N:
N:   Refer to https://bugs.debian.org/884798 for details.

Brltty doesn't currently seem to be using external .m4 files, so I'm not
sure how it would be preferred to include pkg.m4 to get the proper
PKG_PROG_PKG_CONFIG macro.

Samuel