Non-Descriptive DLL Build Errors When Not Using MSVC

[ThomsonTan]

Windows DLL build is only supported for MSVC, but seems not all the places checks it in examples like below. When building DLL on windows with mingw, the error message will be very cryptic like "opentelemetry-cpp::opentelemetry_cpp target was not found". It will be helpful to error out the unsupported scenario instead of postpone it to the error of missing dependence.

opentelemetry-cpp/examples/otlp/CMakeLists.txt

Line 11 in 4b6f52a

if(DEFINED OPENTELEMETRY_BUILD_DLL)