When out-of-src build, at the beginning of a build, `make -f enc.mk
srcs` generates trans C sources under build dir.
On the other hand, enc/trans/*.o were built from trans C sources
generated under srcdir due to the following auto-generated rules from
enc/depend.
```
encsrcdir = ../src/enc
...
enc/trans/big5.$(OBJEXT): $(encsrcdir)/trans/big5.c
```
Therefore, trans C sources are generated twice under srcdir and build
dir during a build.
Ideally, trans C sources have always been built before compilation of
enc/trans/*.o because the source generation is prereq, so making
enc/trans/*.o doesn't trigger trans C source generation and shouldn't
require MINIRUBY as a make arg for enc.mk. However, the second trans C
source gen is unintentionally triggered by enc/trans/*.o, so `make -f
enc.mk libencs` requires MINIRUBY for now.
When no `--with-static-linked-ext`, `make -f enc.mk libencs` is
triggered from common.mk with MINIRUBY, so there is no problem.
But when `--with-static-linked-ext`, libencs should be statically-linked
to ruby, so `make -f enc.mk libencs` is triggered from exts.mk, and
exts.mk invokes it without MINIRUBY.
Therefore, when out-of-src build and with `--with-static-linked-ext`,
the second trans C source gen fails due to missing MINIRUBY.
This issue is deterministically reproducible without -j because
common.mk's `main` rule also has libencs prerequisite.
This patch supresses the second trans C source gen.