documentation for the macro "valid_policy"

checks if the policy compiles

It is defined in the file build.m4.

m4 code:

define(`valid_policy',
`define(`_policy_makefile_increments',`
check_valid_policy:` policy.$(POLICY_VERSION)'`'`
patsubst(`',^,
`	')'
'_policy_makefile_increments)dnl
define(`_policy_tests',_policy_tests`\
		check_valid_policy')')