CMake: Allow building without dpkg

It's not entirely clear if that is useful, but it cannot hurt. We
need to check what is missing to build on foreign systems without
dpkg.

Gbp-Dch: ignore

CMakeLists.txt

@@ -113,10 +113,14 @@ endif()
 set(PACKAGE ${PROJECT_NAME})
 set(PACKAGE_MAIL "APT Development Team <deity@lists.debian.org>")
 
-execute_process(COMMAND dpkg-parsechangelog -SVersion -l${PROJECT_SOURCE_DIR}/debian/changelog
-                OUTPUT_VARIABLE PACKAGE_VERSION OUTPUT_STRIP_TRAILING_WHITESPACE)
-execute_process(COMMAND dpkg-architecture -qDEB_HOST_ARCH
-                OUTPUT_VARIABLE COMMON_ARCH OUTPUT_STRIP_TRAILING_WHITESPACE)
+if (NOT DEFINED PACKAGE_VERSION)
+  execute_process(COMMAND dpkg-parsechangelog -SVersion -l${PROJECT_SOURCE_DIR}/debian/changelog
+                  OUTPUT_VARIABLE PACKAGE_VERSION OUTPUT_STRIP_TRAILING_WHITESPACE)
+endif()
+if (NOT DEFINED COMMON_ARCH)
+  execute_process(COMMAND dpkg-architecture -qDEB_HOST_ARCH
+                  OUTPUT_VARIABLE COMMON_ARCH OUTPUT_STRIP_TRAILING_WHITESPACE)
+endif()
 
 # Configure our configuration headers (config.h and apti18n.h)
 configure_file(CMake/config.h.in ${PROJECT_BINARY_DIR}/include/config.h)