build: ignore pragma warnings with GCC

GCC does not support '#pragma mark'.  Add -Wno-unknown-pragmas if the compiler
supports it.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@205389 91177308-0d34-0410-b5e6-96231b3b80d8
sanitise sign comparisons

This is a mechanical change addressing the various sign comparison warnings that
are identified by both clang and gcc.  This helps cleanup some of the warning
spew that occurs during builds.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@205390 91177308-0d34-0410-b5e6-96231b3b80d8