GH-3234 Add support for custom meta URLs at build time

This is not particularly interesting for non-developers.
Also includes some internal restructuring of URL constants in general.

buildconfig/CMakeLists.txt

@@ -0,0 +1,11 @@
+######## Configure the file with build properties ########
+
+configure_file("${CMAKE_CURRENT_SOURCE_DIR}/BuildConfig.cpp.in" "${CMAKE_CURRENT_BINARY_DIR}/BuildConfig.cpp")
+
+add_library(BuildConfig STATIC
+    BuildConfig.h
+    ${CMAKE_CURRENT_BINARY_DIR}/BuildConfig.cpp
+)
+
+target_link_libraries(BuildConfig Qt5::Core)
+target_include_directories(BuildConfig PUBLIC "${CMAKE_CURRENT_SOURCE_DIR}")