build: passthrough CMAKE_VERBOSE_MAKEFILE to embedded discount

Make the build of the embedded copy of the discount library use the
same verbosity level of the cantor build.

thirdparty/CMakeLists.txt

@@ -9,7 +9,7 @@ ExternalProject_Add(
     discount_project
     URL ${CMAKE_CURRENT_SOURCE_DIR}/discount-2.2.6-patched
     SOURCE_SUBDIR cmake
-    CMAKE_ARGS DISCOUNT_ONLY_LIBRARY DISCOUNT_MAKE_INSTALL
+    CMAKE_ARGS DISCOUNT_ONLY_LIBRARY DISCOUNT_MAKE_INSTALL -DCMAKE_VERBOSE_MAKEFILE=${CMAKE_VERBOSE_MAKEFILE}
     CMAKE_CACHE_ARGS "-DCMAKE_POSITION_INDEPENDENT_CODE:BOOL=true"
     PREFIX ${CMAKE_CURRENT_BINARY_DIR}
     INSTALL_COMMAND ""