Fix build on Linux with conda-forge dependencies

[traversaro]

These are the last two fixes that are required to build Gazebo with conda-forge provided dependencies on Linux without any further modification (verified in https://github.com/traversaro/gazebo-conda-forge-ci/pull/2/checks?check_run_id=2024424151, the test suite has some failures, but that's another story).

The fixes are:

• Do not manually add -std=c++11 to CMAKE_CXX_FLAGS. This is not necessary as the cxx_std_11 compile feature is set for all the libraries in https://github.com/osrf/gazebo/blob/67c43463c87685b7c9d485a9f8d2e6ac760815b9/cmake/GazeboUtils.cmake#L106 and https://github.com/osrf/gazebo/blob/67c43463c87685b7c9d485a9f8d2e6ac760815b9/cmake/GazeboUtils.cmake#L120 . Furthermore, it can create problems when external libraries export other compile features (such as ignition-math6 that export cxx_std_17).
• In RestApi.cc use C++ style cinttypes instead of C style inttypes.h.

[j-rivero]

These are the last two fixes that are required to build Gazebo with conda-forge

Amazing! Thanks Silvio.

I've relaunched the CI on Brew to check that things are fine with clang there. If CI is happy, I'm happy too.

[scpeters]

These are the last two fixes that are required to build Gazebo with conda-forge

Amazing! Thanks Silvio.

I've relaunched the CI on Brew to check that things are fine with clang there. If CI is happy, I'm happy too.

brew CI looks ok