Bug#825617: Should Enable Parallel Builds According to DEB_BUILD_OPTIONS

[殷啟聰]

Package: gradle-debian-helper
Version: 1.1
Severity: wishlist

Gradle supports parallel building through "--parallel" and
"--max-workers". Currently gradle-debian-helper does not honor the
parallel options in DEB_BUILD_OPTIONS and I have to write this:

```
ifneq (,$(filter parallel=%,$(DEB_BUILD_OPTIONS)))
  NUMTHREADS = $(patsubst parallel=%,%,$(filter
parallel=%,$(DEB_BUILD_OPTIONS)))
  GRADLE_FLAGS += --parallel --max-workers=$(NUMTHREADS)
endif
```

It would be great if gradle-debian-helper would do this for us.