Avoid touching the "KDChart" forwarding header at every CMake run

This led to unnecessary recompilations in projects using
KDChart as a submodule.

src/CMakeLists.txt

@@ -148,11 +148,14 @@ ecm_generate_headers(
 )
 # Combine required headers into 1 big convenience header
 set(COMMON_HEADER ${CMAKE_CURRENT_BINARY_DIR}/KDChart/KDChart)
-file(WRITE ${COMMON_HEADER} "// convenience header\n")
+# (use a tmp file and configure_file to avoid touching the header unnecessarily)
+set(COMMON_HEADER_TMP ${COMMON_HEADER}.tmp)
+file(WRITE ${COMMON_HEADER_TMP} "// convenience header\n")
 foreach(_header ${kdchart_HEADERS})
     get_filename_component(_base ${_header} NAME)
-    file(APPEND ${COMMON_HEADER} "#include \"${_base}\"\n")
+    file(APPEND ${COMMON_HEADER_TMP} "#include \"${_base}\"\n")
 endforeach()
+configure_file("${COMMON_HEADER_TMP}" "${COMMON_HEADER}" COPYONLY)
 list(APPEND kdchart_HEADERS "${COMMON_HEADER}")
 configure_file("${COMMON_HEADER}" "${COMMON_HEADER}.h" COPYONLY)
 list(APPEND kdchart_HEADERS "${COMMON_HEADER}.h")