Use standard AC_PROG_INSTALL macro to search for install program,

Use standard AC_PROG_INSTALL macro to search for install program,
instead of our own halfway-there code.  Add AC_STRUCT_TIMEZONE call
to check whether tm_zone exists in struct tm.  Revise reading of template
file so that templates can define any variables they feel like (and,
indeed, can execute arbitrary shell code) rather than being constrained
to a fixed set of variable names.