package/shairport-sync: fix static builds

package/shairport-sync: fix static builds

Because shairport-sync uses AC_CHECK_LIB to find some of the needed
libraries (opensl, alsa, popt...), it fails to bring in the dependencies
of those libraries; for example, -lssl requires -lz.

When doing a dynamically-linked build, those dependent libraries are
pulled in thanks to the DT_NEEDED ELF tags, but those are not available
in static libraries.

Use PKG_CHECK_MODULES instead of AC_CHECK_LIB, and emulate the results
of AC_CJECK_LIB (i.e. set the HAVE_LIBxxx and the LIBS variable
appropriately) to minimise the amount of code touched.

This should fix:
    http://autobuild.buildroot.net/results/1a5/1a56dbff3583bed0d693508dfc16859086846ecd/
    http://autobuild.buildroot.net/results/6d5/6d5a8157592f7a2c9469bf71b0453796ef982ab1/

At the same time, remove a comment at the end of a variable assignment,
since this has proven to cause some problems.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Jörg Krause <jkrause@posteo.de>
Cc: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>