Properly handle the requirement of C++17 at the CMake exported target level (#251)

SDFormat 9 requires the use of C++17 in its public headers, however it does not correctly expose this requirement
in its installed imported targets. In particular, at the moment the requirement of C++17 is only documented via adding the
-std=c++17 flag in the `SDFormat_CXX_FLAGS` variable, but this strategy has two problems:
* It does not work unless a user explicitly pass this flags to its compilation targets, and this is definitely not obvious if it is not using directly SDFormat, but he is linking sdformat transitively through Gazebo 11.
* If a different C++ version is set at the CMake level, it is possible that the `-std=c++17` flag is ignored (it depends what is
the order with which the flags are passed to the compiler).

For CMake consumers, a better strategy is to use the target_compile_features and explicitly mark that
that this library requires the `cxx_std_17` feature, as `PUBLIC` because it also required in the public headers.

As the minimum required version of CMake is 3.10, we can use cxx_std_17 as it is available in CMake 3.10, see https://cmake.org/cmake/help/v3.10/prop_gbl/CMAKE_CXX_KNOWN_FEATURES.html#prop_gbl:CMAKE_CXX_KNOWN_FEATURES .

Signed-off-by: Silvio Traversaro <silvio.traversaro@iit.it>

Author: traversaro

src/CMakeLists.txt

@@ -164,6 +164,7 @@ if (NOT WIN32)
 endif()
 
 sdf_add_library(${sdf_target} ${sources})
+target_compile_features(${sdf_target} PUBLIC cxx_std_17)
 target_link_libraries(${sdf_target} PUBLIC ${IGNITION-MATH_LIBRARIES})
 
 target_include_directories(${sdf_target}