Improve --help documentation of important environment variables.

Improve --help documentation of important environment variables.

* acgeneral.m4 (_AC_INIT_HELP): Direct reader to descriptions
of useful variables at the end of the --help.
(AC_ARG_VAR): Emphasize use of environment variables to override
and/or help the configure script.  Ensure that a given variable
is only documented once in the --help.
(AC_CHECK_LIB): Call AC_ARG_VAR to document and register the
LDFLAGS variable, and...
(AC_CHECK_HEADER): do the same for the CPPFLAGS variable.
* aclang.m4 (AC_PROG_CC): Call AC_ARG_VAR to document and register
the CC and CFLAGS variables, and do the same for...
(AC_PROG_CXX): CXX and CXXFLAGS, and...
(AC_PROG_F77): F77 and FFLAGS.