BUILD.mk: missed in prior commit

BUILD.md

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+# indentlog build details
+
+## Test Coverage
+
+### enable coverage build
+```
+$ cd indentlog
+$ mkdir ccov
+$ cd ccov
+$ cmake -DCODE_COVERAGE=ON ..      # prepares coverage build
+```
+
+### build + generate test coverage
+```
+$ make ccov                                               # builds + runs unit tests
+$ make ccov-all                                         # generates .html report
+```
+
+### view coverage report
+```
+$ firefox
+[navigate to coverage report;  path something like file://home/roland/proj/indentlog/ccov/ccov/all-merged/index.html]
+```
+
+![lcov_output](img/lcov1.png)
+
+## Implementation Notes
+
+- coverage builds creates `.gcno` files alongside `.o` object files
+- running coverage-enabled executables creates/appends to `.gcda` files
+- e.g. see `ccov/utest/CMakeFiles/utest.indentlog.dir`
+- coverage feature enabled globally in top-level `CMakeLists.txt` by:
+```
+include(cmake/code-coverage.cmake)
+add_code_coverage()
+add_code_coverage_all_targets()
+```
+- looks like these need to appear before executable targets are introduced
+- also need to opt-in individual executables,  e.g. in `utest/CMakeLists.txt`:
+```
+target_code_coverage(${SELF_EXECUTABLE_NAME} AUTO ALL)
+```
+- here `AUTO` opts in to the `ccov` target;  `ALL` opts in to the `ccov-all` target