Refactor cmake (#1231)

Fixes #920.

benchmarks/CMakeLists.txt

@@ -0,0 +1,16 @@
+add_executable(clio_benchmark)
+
+target_sources(
+  clio_benchmark
+  PRIVATE # Common
+          Main.cpp
+          Playground.cpp
+          # ExecutionContext
+          util/async/ExecutionContextBenchmarks.cpp
+)
+
+include(deps/gbench)
+
+target_include_directories(clio_benchmark PRIVATE .)
+target_link_libraries(clio_benchmark PUBLIC clio benchmark::benchmark_main)
+set_target_properties(clio_benchmark PROPERTIES RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR})