openmvg: fixup build

The build broke probably after cmake upgrade in b8ac5b6cfb,
but either way - this forcing of old C++ standard seemed bad.
https://hydra.nixos.org/build/225241222/nixlog/2/tail

pkgs/applications/science/misc/openmvg/default.nix

···
   nativeBuildInputs = [ cmake pkg-config ];
 
   cmakeFlags = [
-    "-DCMAKE_CXX_FLAGS=-std=c++11"
     "-DOpenMVG_BUILD_EXAMPLES=${if enableExamples then "ON" else "OFF"}"
     "-DOpenMVG_BUILD_DOC=${if enableDocs then "ON" else "OFF"}"
     "-DTARGET_ARCHITECTURE=generic"