Original commit message from CVS:
Tighten some library checks (check for header files as well as libraries)
Also, clean up structure slightly: put all AC_DEFINEs for feature checks
into the appropriate section at end.  This structure allows user parameter
code which overrides the feature selection to be kept together in a section
after the feature checks but before the AC_DEFINES and AM_CONDITIONALs.