Fix make dist on a clean checkout

cmake with unix makefiles fails to resolve dist dependencies
added from COPY_FROM_BUILD if these dependencies aren't built yet.

This commit replaces the COPY_FROM_BUILD based logic with two new functions
'dist_add_configured' and 'dist_add_generated' to indicate which files should
be included in the dist tarball. The latter also adds a target level dependency
to the dist tarball custom command. Hence the former should
be used for files that get generated during a cmake run while the latter
should be used for files generated as the result of a 'make/ninja-build' run
(like files for which an add_custom_command rule exists).

Note: this commit also temporarily disables the dist target when building
from a tarball (and hence it won't be tested in distcheck either). This
will be handled in a future commit.

bindings/python/CMakeLists.txt

@@ -46,6 +46,8 @@ IF (BUILDING_FROM_VCS)
     ${CMAKE_SOURCE_DIR}/libgnucash/engine/engine-common.i
     ${GNUCASH_CORE_C_INCLUDES}
   )
+  add_custom_target(swig-gnucash-core-c-py DEPENDS swig-gnucash-core)
+  dist_add_generated(swig-gnucash-core-c-py gnucash_core_c.py)
 ELSE()
   SET (SWIG_GNUCASH_CORE_C gnucash_core.c)
   SET (SWIG_GNUCASH_CORE_C_PY ${CMAKE_CURRENT_SOURCE_DIR}/gnucash_core_c.py)