Changes

m4/issm_options.m4: new ISSM_ENABLE_AD macro (–enable-ad support)

* Adds ISSM_ENABLE_AD to handle --enable-ad plus --with-{codi,medi}pack-dir.
* Macro embeds CoDiPack/MediPack include-dirs, lib-dirs and -lcodi/-lmedi
  into AM_CPPFLAGS, AM_LDFLAGS, LIBS so no Makefile.am edits are needed.
* Automatically drops PETSc when AD is on, defines ISSM_USE_AD, and exposes
  AM_CONDITIONAL(USE_AD).
* configure.ac now simply calls ISSM_ENABLE_AD after ISSM_OPTIONS.

Moves all AD logic out of configure.ac and keeps option handling in the
shared macro library.

added enable AD into configure.ac
 (commit: 76d01cc)