build: don't let environment settings perturb build

build: don't let environment settings perturb build

Setting the envvars, LIB_FDATASYNC, LIB_XATTR or LIB_CRYPT
could cause a configure-time and/or build-time malfunction.
Typically, a configure-time function-in-library test is performed
via code like this:

  LIB_VAR=
  AC_SUBST([LIB_VAR])
  prefix_saved_LIBS=$LIBS
    AC_SEARCH_LIBS([FUNC], [LIB_NAME],
                   [test "$ac_cv_search_FUNC" = "none required" ||
                    LIB_VAR=$ac_cv_search_FUNC])
  LIBS=$prefix_saved_LIBS

However, in each of the files affected by this change, the LIB_VAR=
initialization was omitted.  Thus, when set in the environment, its
value would propagate into generated Makefiles when FUNC is not found
in LIB_NAME.
* m4/jm-macros.m4 (coreutils_MACROS): Initialize AC_SUBST'd var
* m4/lib-check.m4 (cu_LIB_CHECK): Likewise.
* m4/xattr.m4 (gl_FUNC_XATTR): Likewise.