* configure.ac: Invoke AC_INIT, AC_OUTPUT without arguments.
* doc/configure.ac: Likewise.
* lib/configure.ac: Likewise.
* src/configure.ac: Invoke AC_INIT, AC_OUTPUT without arguments. Use
AC_CACHE_CHECK, AC_LANG_PUSH/AC_LANG_POP, AC_COMPILE_IFELSE
* tests/configure.ac: Invoke AC_INIT, AC_OUTPUT without arguments. Use
ac_cv_c_compiler_gnu instead of ac_cv_prog_gcc, and
ac_cv_cxx_compiler_gnu instead of ac_cv_prog_gxx. Don't use 'test' with
option '-a'.