Protobuf package: allow building shared library (#7067)

This adds a +shared variant for building shared libraries in the
protobuf package and enables it by default.

var/spack/repos/builtin/packages/protobuf/package.py

@@ -41,6 +41,9 @@ class Protobuf(CMakePackage):
     # does not build with CMake:
     # version('2.5.0', '9c21577a03adc1879aba5b52d06e25cf')
 
+    variant('shared', default=True,
+            description='Enables the build of shared libraries')
+
     depends_on('zlib')
 
     conflicts('%gcc@:4.6')  # Requires c++11
@@ -62,6 +65,7 @@ def fetch_remote_versions(self):
 
     def cmake_args(self):
         args = [
+            '-DBUILD_SHARED_LIBS=%s' % int('+shared' in self.spec),
             '-Dprotobuf_BUILD_TESTS:BOOL=OFF',
             '-DCMAKE_POSITION_INDEPENDENT_CODE:BOOL=ON'
         ]