src: Build the Emoji dictionaries in parallel
src: Build the Emoji dictionaries in parallel

Instead of building Emoji dictionaries src/dicts/emoji-*.dict in sequence, a
pattern rule is specified for them. The make -jN option builds the
dictionaries in parallel.

The GNU make extensions like pattern rule and patsubst function are used for
it. But src/Makefile.am has had other GNU make extensions for a while, so
using more extensions should not make portability worse.

BUG=https://github.com/ibus/ibus/pull/2209

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