Cmake std fix

[andrewfb]

I investigated this more and discovered that CMake's export() command doesn't propagate INTERFACE_CXX_STANDARD or related properties to consuming applications. The solution appears to be capturing Cinder's C++ standard and extensions settings in cinderConfig.cmake and explicitly applying them to all applications via ci_make_app(). You'll note that the implementation uses a priority chain: user's explicit CMAKE_CXX_STANDARD override > Cinder's standard > default C++17. Extensions default to OFF (preventing the "namespace linux" issue) unless explicitly enabled with CMAKE_CXX_EXTENSIONS=ON.

Part of what this PR changes is that overriding the C++ standard - including specifying gnu++, should work now, and Cinder can actually build despite the linux definition with an additional flag.

These are the combinations I tested:

Test Scenario	Compiler	Cinder Build Command	App Build Command	Result
Default	g++, clang++	cmake ..	cmake ..	Both use -std=c++17
Cinder C++20, App inherits	g++, clang++	cmake .. -DCMAKE_CXX_STANDARD=20	cmake ..	Both use -std=c++20
Cinder C++23, App inherits	g++, clang++	cmake .. -DCMAKE_CXX_STANDARD=23	cmake ..	Both use -std=c++23
Cinder default, App C++20	g++, clang++	cmake ..	cmake .. -DCMAKE_CXX_STANDARD=20	Cinder -std=c++17, App -std=c++20
Cinder C++20, App C++23	g++, clang++	cmake .. -DCMAKE_CXX_STANDARD=20	cmake .. -DCMAKE_CXX_STANDARD=23	Cinder -std=c++20, App -std=c++23
Cinder gnu++17, App inherits	g++, clang++	cmake .. -DCMAKE_CXX_STANDARD=17 -DCMAKE_CXX_EXTENSIONS=ON -DCMAKE_CXX_FLAGS="-Ulinux"	cmake .. -DCMAKE_CXX_FLAGS="-Ulinux"	Both use -std=gnu++17 with -Ulinux
Cinder gnu++20, App inherits	g++, clang++	cmake .. -DCMAKE_CXX_STANDARD=20 -DCMAKE_CXX_EXTENSIONS=ON -DCMAKE_CXX_FLAGS="-Ulinux"	cmake .. -DCMAKE_CXX_FLAGS="-Ulinux"	Both use -std=gnu++20 with -Ulinux
Cinder default, App gnu++17	g++, clang++	cmake ..	cmake .. -DCMAKE_CXX_STANDARD=17 -DCMAKE_CXX_EXTENSIONS=ON -DCMAKE_CXX_FLAGS="-Ulinux"	Cinder -std=c++17, App -std=gnu++17 with -Ulinux
Cinder gnu++17, App c++20	g++, clang++	cmake .. -DCMAKE_CXX_STANDARD=17 -DCMAKE_CXX_EXTENSIONS=ON -DCMAKE_CXX_FLAGS="-Ulinux"	cmake .. -DCMAKE_CXX_STANDARD=20 -DCMAKE_CXX_EXTENSIONS=OFF	Cinder -std=gnu++17 with -Ulinux, App -std=c++20
Cinder c++17, App gnu++20	g++, clang++	cmake ..	cmake .. -DCMAKE_CXX_STANDARD=20 -DCMAKE_CXX_EXTENSIONS=ON -DCMAKE_CXX_FLAGS="-Ulinux"	Cinder -std=c++17, App -std=gnu++20 with -Ulinux
Reject C++14	g++, clang++	cmake .. -DCMAKE_CXX_STANDARD=14	N/A	❌ Correctly rejected
Reject C++11	g++, clang++	cmake .. -DCMAKE_CXX_STANDARD=11	N/A	❌ Correctly rejected

As a bonus this includes a small fix to build with C++20. Hoping this addresses @PetrosKataras' issue seen against #2361