google benchmark: Add variant with libpfm4 (#42620)

Signed-off-by: Steven Hahn <hahnse@ornl.gov>

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

@@ -44,14 +44,24 @@ class Benchmark(CMakePackage):
         description="The build type to build",
         values=("Debug", "Release", "RelWithDebInfo", "MinSizeRel", "Coverage"),
     )
+    variant(
+        "performance_counters",
+        default=True,
+        when="@1.5.4:",
+        description="Enable performance counters provided by libpfm",
+    )
 
     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")
     depends_on("cmake@3.5.1:", type="build", when="@1.5.0:")
+    depends_on("libpfm4", type=("build", "link"), when="+performance_counters")
 
     def cmake_args(self):
         # No need for testing for the install
-        args = ["-DBENCHMARK_ENABLE_TESTING=OFF"]
+        args = [
+            self.define("BENCHMARK_ENABLE_TESTING", False),
+            self.define_from_variant("BENCHMARK_ENABLE_LIBPFM", "performance_counters"),
+        ]
         return args
 
     def patch(self):