package/librtlsdr: fix version fetching when code is extracted from tarball

package/librtlsdr: fix version fetching when code is extracted from tarball

librtlsdr currently fails to build on the autobuilders, as it fails
for out of tree builds. Indeed, there is some CMake logic in librtlsdr
that determines the version using Git. This works fine when librtlsdr
is fetched from Git of course. But in the context of Buildroot,
librtlsdr is extracted from a tarball.

For an in-tree build, the "git describe" invocation goes all the way
up to the Buildroot .git/ metadata, and uses that as the librtlsdr
version (it's of course wrong, but the build works). In an out-of-tree
build, there is no parent directory with .git/ metadata, so Git fails,
the VERSION variable is empty and later CMake aborts the build because
of that.

We fix that by adjusting the version retrieving logic to only use Git
if a .git/ metadata folder is found at the root of the librtlsdr
source tree. The patch has been submitted upstream.

Fixes:

  http://autobuild.buildroot.net/results/ea52be1da8ed03272db06679d5a0a441ffe6ea0c/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>