talloc: Fix linker flags on darwin.

Before this patch, building talloc on darwin (OS X 10.8.5) results in:

gcc -dynamiclib -Wl,-search_paths_first -undefined error -o libtalloc.dylib.2.0.1 ./talloc.o  ./libreplace/replace.o ./libreplace/snprintf.o ./libreplace/getpass.o ./libreplace/strptime.o   -install_namelibtalloc.dylib.2
gcc: error: unrecognized command line option '-install_namelibtalloc.dylib.2'
make: *** [libtalloc.dylib.2.0.1] Error 1

Setting SONAMEFLAG to either "-install_name " (note trailing space), or
"-Wl,-install_name," results in a successful build. The latter seems
more clear.
Merge pull request #4557 from pmahoney/talloc-darwin

talloc: Fix linker flags on darwin.