target_sources: Interpret relative paths as relative to the calling directory

Previously the command considered non-absolute source file paths relative to
the associated target on the LHS. This causes problems in incremental builds
where files are added from subdirectories and forces users to workaround by
manually converting to absolute paths. Change this to enable more intuitive
usage by projects.

Fixes #17981

Tests/RunCMake/TargetSources/RelativePathInSubdirPrivate/subdir_empty_2.cpp

@@ -0,0 +1,7 @@
+#ifdef _WIN32
+__declspec(dllexport)
+#endif
+  int empty()
+{
+  return 0;
+}