diff --git a/cmake/setup.cmake b/cmake/setup.cmake
index 4008030..0a710b6 100644
--- a/cmake/setup.cmake
+++ b/cmake/setup.cmake
@@ -48,6 +48,35 @@ if(NOT HAVE_STDINT_H)
   add_compile_definitions(NO_STDINT_H)
 endif()
 
+# Try to normalise source file pathnames as seen in __FILE__ (e.g.
+# assertion failure messages). Partly to avoid bloating the binaries
+# with file prefixes like /home/simon/stuff/things/tmp-7x6c5d54/, but
+# also to make the builds more deterministic - building from the same
+# source should give the same binary even if you do it in a
+# differently named temp directory.
+function(map_pathname src dst)
+  if(CMAKE_SYSTEM_NAME MATCHES "NestedVM")
+    # Do nothing: the NestedVM gcc is unfortunately too old to support
+    # this option.
+  elseif(CMAKE_C_COMPILER_ID MATCHES "Clang" AND
+      CMAKE_C_COMPILER_FRONTEND_VARIANT MATCHES "MSVC")
+    # -fmacro-prefix-map isn't available as a clang-cl option, so we
+    # prefix it with -Xclang to pass it straight through to the
+    # underlying clang -cc1 invocation, which spells the option the
+    # same way.
+    set(CMAKE_C_FLAGS
+      "${CMAKE_C_FLAGS} -Xclang -fmacro-prefix-map=${src}=${dst}"
+      PARENT_SCOPE)
+  elseif(CMAKE_C_COMPILER_ID MATCHES "GNU" OR
+      CMAKE_C_COMPILER_ID MATCHES "Clang")
+    set(CMAKE_C_FLAGS
+      "${CMAKE_C_FLAGS} -fmacro-prefix-map=${src}=${dst}"
+      PARENT_SCOPE)
+  endif()
+endfunction()
+map_pathname(${CMAKE_SOURCE_DIR} /puzzles)
+map_pathname(${CMAKE_BINARY_DIR} /build)
+
 include(icons/icons.cmake)
 
 # The main function called from the top-level CMakeLists.txt to define