libssh: fix build with musl libc

libssh: fix build with musl libc

libssh uses GNU extensions to the glob() API. Update the cmake test to
take that into account so that glob() is not used with musl libc.

Fixes:
http://autobuild.buildroot.net/results/0c2/0c2d17316fd6bd2bf1359e23a2a1273fa349cf2a/
http://autobuild.buildroot.net/results/936/936abac5362b33980fd1efe8b830409ee2f86f6b/
http://autobuild.buildroot.net/results/51b/51b1b136ab4209ee443c1b450a932341b2ff81b7/

Cc: Scott Fan <fancp2007@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>