Implement coverage option

To use pass -DCMAKE_BUILD_TYPE=Debug or Asan -DCOVERAGE=ON and build as
usual, then do ninja lcov-initialize && ninja check && ninja
lcov-collect. The result will be a directory, <Builddir>/Coverage
containing lcov tracefiles, including an aggregate file gnucash.info
which you can use for further processing. It will also report an overall summary.
Note that only C/C++ files are included.

There's one more target, lcov-generate-html, that you can run after
lcov-collect. It will generate a simple website in
<Builddir>/Coverage-HTML showing coverage by source directory (the
directories in <Builddir> have coverage for generated files). Each
directory path is a clickable link to a page that shows coverage for
each source file; the filenames link to a page for each showing which
lines have been exercised.

libgnucash/backend/dbi/CMakeLists.txt

@@ -43,6 +43,10 @@ if (WITH_SQL)
     set_target_properties (gncmod-backend-dbi PROPERTIES INSTALL_NAME_DIR "${CMAKE_INSTALL_FULL_LIBDIR}/gnucash")
   endif()
 
+  if (COVERAGE)
+    add_coverage_target(gncmod-backend-dbi)
+  endif()
+
   install(TARGETS gncmod-backend-dbi
     LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}/gnucash
     ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}/gnucash

libgnucash/backend/sql/CMakeLists.txt

@@ -80,5 +80,9 @@ if(WITH_SQL)
     ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
     RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR})
 
+  if (COVERAGE)
+    add_coverage_target(gnc-backend-sql)
+  endif()
+
   # No headers to install
 endif()

libgnucash/backend/xml/CMakeLists.txt

@@ -96,6 +96,10 @@ install(TARGETS gnc-backend-xml-utils
   ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
   RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR})
 # No headers to install
+if (COVERAGE)
+  add_coverage_target(gnc-backend-xml-utils)
+endif()
+
 
 # ----
 
@@ -119,6 +123,10 @@ if (APPLE)
   set_target_properties (gncmod-backend-xml PROPERTIES INSTALL_NAME_DIR "${CMAKE_INSTALL_FULL_LIBDIR}")
 endif()
 
+if (COVERAGE)
+  add_coverage_target(gncmod-backend-xml)
+endif()
+
 install(TARGETS gncmod-backend-xml
   LIBRARY DESTINATION ${LIB_DIR}
   ARCHIVE DESTINATION ${LIB_DIR}