Change compilation flags to support clang >3.4

removing the unsupported flag "-fno-reorder-blocks". In addition leverage the
fact that the minimum gcc version is 4.8 to provide a relwithdebinfo build
with the Og optimisation level.
From https://gcc.gnu.org/gcc-4.8/changes.html:
"A new general optimization level, -Og, has been introduced. It addresses the
need for fast compilation and a superior debugging experience while providing
a reasonable level of runtime performance. Overall experience for development
should be better than the default optimization level -O0."