cmake: cxx_std_14 -> cxx_generic_lambdas

Apparently, this is compatible with older versions of cmake, while having the same effect. Apparently, the cmake people spent some time refuling to let people specify C++ standard versions and instead insisted that they specify specific features instead. They did not see the light until cmake 3.8, but that's too new for us to require yet, I guess.

c++/cmake/CapnProtoTargets.cmake

@@ -136,7 +136,8 @@ function(_capnp_import_pkg_config_target target)
   set_target_properties(CapnProto::${target} PROPERTIES
     ${imported_soname_property}
     IMPORTED_LOCATION "${target_location}"
-    INTERFACE_COMPILE_FEATURES "cxx_std_14"
+    # TODO(cleanup): Use cxx_std_14 once it's safe to require cmake 3.8.
+    INTERFACE_COMPILE_FEATURES "cxx_generic_lambdas"
     INTERFACE_COMPILE_OPTIONS "${${target}_CFLAGS_OTHER}"
     INTERFACE_INCLUDE_DIRECTORIES "${${target}_INCLUDE_DIRS}"
 

c++/src/kj/CMakeLists.txt

@@ -71,7 +71,8 @@ set(kj-std_headers
 )
 add_library(kj ${kj_sources})
 add_library(CapnProto::kj ALIAS kj)
-target_compile_features(kj PUBLIC cxx_std_14)
+# TODO(cleanup): Use cxx_std_14 once it's safe to require cmake 3.8.
+target_compile_features(kj PUBLIC cxx_generic_lambdas)
 
 if(UNIX AND NOT ANDROID)
   target_link_libraries(kj PUBLIC pthread)