docs: further specify that CFLAGS and friends are only for distro packagers [skip ci]

docs/markdown/howtox.md

@@ -52,7 +52,9 @@ executable(..., dependencies : thread_dep)
 
 ## Set extra compiler and linker flags from the outside (when e.g. building distro packages)
 
-The behavior is the same as with other build systems, with environment variables during first invocation.
+The behavior is the same as with other build systems, with environment
+variables during first invocation. Do not use these when you need to rebuild
+the source
 
 ```console
 $ CFLAGS=-fsomething LDFLAGS=-Wl,--linker-flag meson <options>