Add option to use a user-provided Yasm with autotools build-system.
Document Yasm related options.
Remove libtool hacks.
Remove libtool hacks for C++.
Fix typos in assembly file names.
Fix for PIC code on Darwin.

Originally fixed at http://trac.sagemath.org/ticket/13137
Fix for Intel Core and Pentium4 Prescott CPUs.

Originally fixed at http://trac.sagemath.org/ticket/14172
Fix for Sun Studio compiler.
Properly quote define so that m4 does not fail.
Support *-w64-mingw* and *-*-cygwin*.
Use AC_CONFIG_ITEMS macros.
Discard options when configuring Yasm.
Restore Yasm 1.2.0 vanilla sources.
Fix copy of config.* scripts for Yasm.
Fix for ABI testing on Cygwin/MinGW.
Fix fo headers generation.
Fix for MPIR_AS.
Let the Win64 renaming trick work with the new AC_OUTPUT macro.
Put back a corrected version of some arguably useless code.
Harmonize PROLOGUE in Win64 assembly files.

This breaks non-generic builds with MinGW.