Prioritise unwind_INCLUDE_DIRS path

[masaori335]

Problem

traffic_crashlog uses nongnu libunwind and including libunwind.h and libunwind-ptrace.h header. OTOH, LLVM has its own libunwind.h. Sometime, CMake pickup LLVM's libunwind.h and making error like below.

In file included from /workspace/src/traffic_crashlog/backtrace.cc:39:
/usr/include/libunwind-ptrace.h:57:8: error: unknown type name 'unw_accessors_t'; did you mean 'unw_cursor_t'?
extern unw_accessors_t _UPT_accessors;
       ^
/opt/llvm/include/libunwind.h:78:29: note: 'unw_cursor_t' declared here
typedef struct unw_cursor_t unw_cursor_t;
                            ^

Approach

Force CMake to set ${unwind_INCLUDE_DIRS} first and pickup nongnu libunwind's libunwind.h header.

[masaori335]

This works if libunwind.h is in specific path like /opt/libunwind/include/libunwind.h. However, if it's in default search path like /usr/include/libunwind.h, this doesn't work. Does anybody know better way?

[cmcfarlen]

Cherry-picked to 10.0.x