CMake improvements - expose ifs in options

[Xeverous]

The following switch is not visible in cmake -L or in the cmake-gui. A library user is not aware of being able to disable examples unless the variable is added blindly.

elements/CMakeLists.txt

Lines 19 to 21 in 9e73461

	if (NOT ELEMENTS_NO_EXAMPLES)
	add_subdirectory(examples)
	endif()

With modern CMake, it is recommended to do such thing instead:

option(ELEMENTS_BUILD_EXAMPLES "build Elements library examples" ON)

if (NOT ELEMENTS_BUILD_EXAMPLES) 
    add_subdirectory(examples) 
endif() 

Would you like a PR for this?

[djowel]

Would you like a PR for this?

Absolutely!

[Xeverous]

Which branch should I target with PRs? develop?

[djowel]

Which branch should I target with PRs? develop?

develop please