Pass args to pip-compile. Hint to force make.

Makefile

@@ -8,5 +8,5 @@ pip_compile: $(PIP_DEPENDENCY_TARGETS)
 
 # All `pip` dependency files depend on their corresponding `.in` file and the base `requirements.in`.
 $(PIP_DEPENDENCY_DIR)/%.txt: $(PIP_DEPENDENCY_DIR)/%.in $(PIP_DEPENDENCY_DIR)/requirements.in
-	pip-compile --output-file $@ $<
+	pip-compile --output-file=$@ ${ARGS} $<
 

README.md

@@ -6,7 +6,7 @@ KPI
 
 Python Dependencies
 -------------------
-Python dependencies are managed with `pip-compile` and `pip-sync` from the [`pip-tools`](https://github.com/jazzband/pip-tools/) package. The dependencies are listed in [`dependencies/pip/`](./dependencies/pip/), with core requirements in [`dependencies/pip/requirements.in`](./dependencies/pip/requirements.in). You may use `pip` directly with one of the compiled `dependencies/pip/*.txt` files, but consider using instead the `pip-sync`. **Do not** add new dependencies directly to the *compiled* `dependencies/pip/*.txt` files; instead, update the relevant the *source* `dependencies/pip/*.in` file(s), and execute `make pip_compile` after any changes.
+Python dependencies are managed with `pip-compile` and `pip-sync` from the [`pip-tools`](https://github.com/jazzband/pip-tools/) package. The dependencies are listed in [`dependencies/pip/`](./dependencies/pip/), with core requirements in [`dependencies/pip/requirements.in`](./dependencies/pip/requirements.in). You may use `pip` directly with one of the compiled `dependencies/pip/*.txt` files, but consider using instead the `pip-sync`. **Do not** add new dependencies directly to the *compiled* `dependencies/pip/*.txt` files; instead, update the relevant the *source* `dependencies/pip/*.in` file(s), and execute `make pip_compile` after any changes. You can pass arguments to `pip-compile` with e.g. `make pip_compile ARGS='--upgrade-package=xlwt'`. To force building, use `make --always-make ...`.
 
 Ubuntu 16.04 `apt` Dependencies
 -------------------------------