package/czmq: fix static linking

package/czmq: fix static linking

Currently, building czmq statically fails for some architectures with undefined
references to the math library:
  /usr/powerpc-buildroot-linux-uclibc/sysroot/usr/lib/libstdc++.a(compatibility-ldbl.o): In function `std::tr1::hash<long double>::operator()(long double) const':
  compatibility-ldbl.cc:(.text._ZNKSt3tr14hashIeEclEe+0x3c): undefined reference to `frexpl'

It turns out that this is actually a problem with libstdc++. Depending on the
arch, some functions will be in compatibility-ldbl.o. If you use any of these
functions, that file will be linked in. And when that file is linked in, all
the _other_ functions, which you actually don't use, are also linked in. And
one of these functions is the template function hash<long double>::operator(),
which uses frexpl.

Requiring czmq to link with -lm is a bit weird, given that neither itself,
nor any of its dependencies really need it...

So ideally it should be fixed in libstdc++ itself. However, as upstream
accepted a patch to link with -lm, we use this patch to fix static linking.

Fixes:
http://autobuild.buildroot.net/results/206/2061238affb685db998100a56c6571538d089a5b/
http://autobuild.buildroot.net/results/caf/caf002d213c9dd09df07858c5c199411ca8ddfa3/
http://autobuild.buildroot.net/results/568/568ac92d2f6d0604b9bf354ea9dca0a7cb5341f6/
http://autobuild.buildroot.net/results/945/9452982fc14f579c61b45a1a189db5f89aab3b80/

Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>