core-utils - make the swig generated source file dependent on the core-utils headers

That should trigger a regeneration of these swig sources if
any of the header files change.
This is done via a small macro that can be reused for other wrappers as well.

Note
cmake 3.15 introduces a 'FILTER' generator expression
that might allow us to do something like the following:
$<FILTER:$<TARGET_PROPERTY:baselib,SOURCES>,INCLUDE,"*.h[pp]?$">
I toyed briefly with that idea but it currently has two issues:
1. 3.15 is newer than our current minimum cmake requirement, so we can't
   depend in that feature yet.
2. the sources are relative to *their* source directory, which
   is different from the one in which the wrappers are generated
   So they should still be properly transformed into absolute paths

common/cmake_modules/GncAddSwigCommand.cmake

@@ -90,3 +90,25 @@ macro (gnc_add_swig_python_command _target _out_var _py_out_var _output _py_outp
     )
     add_custom_target(${_target} ALL DEPENDS ${outfile} ${py_outfile} ${CMAKE_SOURCE_DIR}/common/base-typemaps.i ${_input} ${ARGN})
 endmacro()
+
+
+# The swig wrappers need to know the header files ("the interface")
+# for the library they are wrapping.
+# We can extract those from the target's SOURCES property
+# Using a few ordinary cmake commands
+macro (gnc_swig_extract_header_files _target _variable)
+    set(${_variable} "")
+    get_target_property(_headers ${_target} SOURCES)
+    if(_headers)
+        list(FILTER _headers INCLUDE REGEX ".*[.]h(pp)?$")
+        get_target_property(_srcdir ${_target} SOURCE_DIR)
+        foreach (_header  ${_headers})
+            if(NOT IS_ABSOLUTE "${_header}")
+                set(_header_abs "${_srcdir}/${_header}")
+            else()
+                set(_header_abs "${_header}")
+            endif()
+            list (APPEND ${_variable} "${_header_abs}")
+        endforeach ()
+    endif()
+endmacro()