test(coverage): a reproducible coverage preset, report and floor check

cmake/plot_options.cmake

@@ -20,6 +20,24 @@ option(PLOT_INSTALL "Generate install and export rules (find_package(plot))" ON)
 set(PLOT_SANITIZE "" CACHE STRING
     "Sanitizers to build with, as the list a -fsanitize= flag takes (e.g. address); empty for none")
 
+# The reproducible coverage report lives in tools/coverage.sh, and its `coverage`
+# CMake target is offered only in a tree that is actually instrumented -- the
+# target re-runs the capture, and a report over an uninstrumented build is a
+# lie with numbers in it. The linux-coverage preset sets this; the other three
+# presets leave it off, so their target lists, configure time and build product
+# are exactly what they were before this option existed.
+option(PLOT_COVERAGE "Add the `coverage` target (needs a --coverage build tree)" OFF)
+if(PLOT_COVERAGE AND NOT CMAKE_CXX_FLAGS MATCHES "--coverage")
+  message(FATAL_ERROR
+    "plot: PLOT_COVERAGE=ON needs a --coverage build (CMAKE_CXX_FLAGS is \"${CMAKE_CXX_FLAGS}\"). "
+    "Configure with --preset linux-coverage, or drop -DPLOT_COVERAGE=ON.")
+endif()
+if(PLOT_COVERAGE AND NOT PLOT_BUILD_TESTS)
+  message(FATAL_ERROR
+    "plot: PLOT_COVERAGE=ON needs PLOT_BUILD_TESTS=ON -- the coverage report measures what "
+    "the test suite executes, so a coverage tree with no suite has nothing to report.")
+endif()
+
 # Sanitizer flags go on every target in this project, fetched third-party sources
 # included: a partially instrumented build reports as loudly about the
 # instrumentation gap as about any real bug. The flags are one -fsanitize= list