Update x86[-64] assembler recommendations

NASM 2.11.08 has a bug that prevents it from properly assembling a
macho64 version of libjpeg-turbo (the resulting binary generates corrupt
images.)  2.11.09 works properly.  YASM also works properly and has been
a supported alternative since libjpeg-turbo 1.2.

diff --cc BUILDING.txt
index 986de22b94474dab2940b8387a45257ce1aa2561,97bd521a3f0e97b2e7328e239576868999b03e7c..169a1be566420d5b23953ceb3c35c984cf9cb8b4
--- 1/BUILDING.txt
--- 2/BUILDING.txt
+++ b/BUILDING.txt
@@@ -10,15 -10,15 +10,18 @@@ Build Requirement
  -- autoconf 2.56 or later
  -- automake 1.7 or later
  -- libtool 1.4 or later
 +   * If using Xcode 4.3 or later on OS X, autoconf and automake are no longer
 +     provided.  The easiest way to obtain them is from MacPorts
 +     (http://www.macports.org/).
  
- -- NASM (if building x86 or x86-64 SIMD extensions)
-    * 0.98, or 2.01 or later is required for a 32-bit build
-    * NASM 2.00 or later is required for a 64-bit build
-    * NASM 2.07 or later is required for a 64-bit build on OS X.  This can be
-      obtained from MacPorts (http://www.macports.org/).
+ -- NASM or YASM (if building x86 or x86-64 SIMD extensions)
+    * NASM 0.98, or 2.01 or later is required for an x86 build (0.99 and 2.00 do
+      not work properly with libjpeg-turbo's x86 SIMD code.)
+    * NASM 2.00 or later is required for an x86-64 build.
+    * NASM 2.07, or 2.11.09 or later is required for an x86-64 Mac build
+      (2.11.08 does not work properly with libjpeg-turbo's x86-64 SIMD code when
+      building macho64 objects.)  NASM or YASM can be obtained from MacPorts
+      (http://www.macports.org/).
  
     The binary RPMs released by the NASM project do not work on older Linux
     systems, such as Red Hat Enterprise Linux 4.  On such systems, you can