package/ibrcommon: fix basename call under musl

package/ibrcommon: fix basename call under musl

Fixes:
http://autobuild.buildroot.net/results/9c3/9c3889fe80e58b7a35c242332f993992044d2f6b/

ibrcommon was relying on a glibc extension to basename(3) which allowed it
to accept a const char* parameter (instead of char*). Fixed by testing
for __GLIBC__; if not defined (such as when musl is in use) a fallback
block is used which creates a temporary copy of the path.

Signed-off-by: Brendan Heading <brendanheading@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>