* doc/automake.texi (Conditionals): Split in two sections, "Usage"

* doc/automake.texi (Conditionals): Split in two sections, "Usage"
and "Portability", and add a third one, "Limits" to explain how
conditional definitions inside multi-lines definitions can be
handled.
* automake.in (handle_options): Do not assume that
AUTOMAKE_OPTIONS is defined in TRUE, but diagnose conditional
definitions of AUTOMAKE_OPTIONS.
Report from Bas Wijnen.
* tests/amopt.test: New test.
* tests/Makefile.am (TESTS): Add it.