package/protobuf: needs dynamic libraries

package/protobuf: needs dynamic libraries

Eventhough it should be theoretically possible to build protobuf in
static-only, configure.ac includes an m4 macro, ACX_PTHREAD defined in
m4/acx_pthread.m4, which forcibly checks for threads *with* shared libs,
and is completely broken for static-only (as it forces -shared whatever
the user selection), ending up with these configure results:

    checking for the pthreads library -lpthreads... no
    checking whether pthreads work without any flags... no
    checking whether pthreads work with -Kthread... no
    checking whether pthreads work with -kthread... no
    checking for the pthreads library -llthread... no
    checking whether pthreads work with -pthread... yes
    checking for joinable pthread attribute... PTHREAD_CREATE_JOINABLE
    checking if more special flags are required for pthreads... no
    checking whether to check for GCC pthread/shared inconsistencies... yes
    checking whether -pthread is sufficient with -shared... no
    checking whether -lpthread fixes that... no
    checking whether -lc_r fixes that... no
    configure: WARNING: Impossible to determine how to use pthreads with shared libraries
    checking whether what we have so far is sufficient with -nostdlib... no
    checking whether -lpthread saves the day... no
    configure: WARNING: Impossible to determine how to use pthreads with shared libraries and -nostdlib

Fixing this macro is far from trivial; protobuf in a static-only
scenario is probably not too common. So, just disable protobuf for
static-only builds.

Fixes:
    http://autobuild.buildroot.org/results/3ef/3efb86c7e8ec2db5d953d634470cafae79bd34cf/
    http://autobuild.buildroot.org/results/96a/96ae1108fc3193df2a93a779057130b774379655/
    http://autobuild.buildroot.org/results/00c/00c29795980319d38823eec1301e9ebd860ebd2a/
    ...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Nimai Mahajan <nimaim@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>