use both LD_RUN_PATH and -R on Solaris

I'm doing this deliberately.  The -R takes precedence and we want to
have that on the linker line by default.  However, some gems mutate
LDFLAGS and drop the -R that we need (e.g. ruby-shadow), in that case
we 'failsafe' back to asserting the rpath via LD_RUN_PATH.