benchmark: enable shared libraries by default (#47368)

* benchmark: enable shared libraries by default

The existing behaviour of Google Benchmark yiels static objects which
are of little use for most projects. This PR changes the spec to use
dynamic libraries instead.

* Add shared variant

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

@@ -14,6 +14,7 @@ class Benchmark(CMakePackage):
     git = "https://github.com/google/benchmark.git"
 
     license("Apache-2.0")
+    maintainers("stephenswat")
 
     # first properly installed CMake config packages in
     # 1.2.0 release: https://github.com/google/benchmark/issues/363
@@ -54,6 +55,9 @@ class Benchmark(CMakePackage):
         when="@1.5.4:",
         description="Enable performance counters provided by libpfm",
     )
+    variant(
+        "shared", default=True, sticky=True, description="Build the libraries as shared objects"
+    )
 
     depends_on("cmake@2.8.11:", type="build", when="@:1.1.0")
     depends_on("cmake@2.8.12:", type="build", when="@1.2.0:1.4")
@@ -64,6 +68,7 @@ def cmake_args(self):
         # No need for testing for the install
         args = [
             self.define("BENCHMARK_ENABLE_TESTING", False),
+            self.define_from_variant("BUILD_SHARED_LIBS", "shared"),
             self.define_from_variant("BENCHMARK_ENABLE_LIBPFM", "performance_counters"),
         ]
         return args