Implement application level option in esmxBuild.yaml to control ESMX provided compiler flags

[theurich]

PR #399 removed the compiler options set by ESMX from the top level CMakeLists.txt file. As per discussion #399 (comment), we need to provide an application level option in esmxBuild.yaml that provides user control over ESMX provided compiler flags. This way, ESMX can once again provide these useful compiler options to the user.