[libstdc++] Build system cleanup [4/n]

[Kelley Cook]

This takes advantage of a feature introduced with aclocal 1.8 which 
automatically m4_include all the required macro files.  This gets rid of 
the manual includes in configure.ac and acinclude.m4 as well as the 
libtool dance that GCC used to use.

It is noteworthy that this change results in no change to the generated 
configure file while the only change to Makefile.in is the proper 
recognition of libtool.m4 and crossconfig.m4 as dependencies for aclocal.m4.

GCC's other subdirectories using automake are using already using this.

Bootstrapped along with the other changes on i686-pc-cygwin

OK to install?
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