osmesa: link with libunwind if enabled (v2)

osmesa: link with libunwind if enabled (v2)

Fixes linking error in libOSmesa when using libunwind.

CXXLD    libOSMesa.la
src/gallium/auxiliary/.libs/libgallium.a(u_debug_stack.o): In function `symbol_name_cached':
./src/gallium/auxiliary/util/u_debug_stack.c:87: undefined reference to `_ULx86_64_get_proc_name'
src/gallium/auxiliary/.libs/libgallium.a(u_debug_stack.o): In function `debug_backtrace_capture':
./src/gallium/auxiliary/util/u_debug_stack.c:114: undefined reference to `_Ux86_64_getcontext'
./src/gallium/auxiliary/util/u_debug_stack.c:115: undefined reference to `_ULx86_64_init_local'
./src/gallium/auxiliary/util/u_debug_stack.c:117: undefined reference to `_ULx86_64_step'
./src/gallium/auxiliary/util/u_debug_stack.c:123: undefined reference to `_ULx86_64_get_reg'
./src/gallium/auxiliary/util/u_debug_stack.c:124: undefined reference to `_ULx86_64_get_proc_info'
./src/gallium/auxiliary/util/u_debug_stack.c:120: undefined reference to `_ULx86_64_step'
collect2: error: ld returned 1 exit status

v2 : Fixes title and adds the original error it is fixing.

Signed-off-by: Alexandre Demers <alexandre.f.demers@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>