Fix configure when a toolchain sets CMAKE_CXX_STANDARD, release 0.9.2

Found by hunter's CI on cpp-pm/hunter!860: every one of the 14 platform jobs failed to configure cppduals.

CMake Error at CMakeLists.txt:44 (set_property):
  set_property could not find CACHE variable CMAKE_CXX_STANDARD.

Hunter's toolchains set CMAKE_CXX_STANDARD as a normal variable, so the if (NOT CMAKE_CXX_STANDARD) branch that creates the cache entry never runs, and the later set_property(CACHE ...) has nothing to attach the drop-down list to. A -DCMAKE_CXX_STANDARD= on the command line always creates a cache entry, which is why this never showed up here.

Guarded with if (DEFINED CACHE{CMAKE_CXX_STANDARD}). Reproduced with a two-line toolchain file, which now configures; a plain configure still shows the 17/20 drop-down.

Version bumped to 0.9.2, because the three registry recipes need a release that contains the fix.