BUILDING.txt: Clarify NASM build requirements

The version requirements only apply to NASM (not YASM.)  Also, 2.11.09
was never actually released (the first release containing the OS X fix
is 2.12.)

BUILDING.txt

@@ -15,13 +15,13 @@ Build Requirements
      (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/).
+   * If using 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.)
+   * If using NASM, 2.00 or later is required for an x86-64 build.
+   * If using NASM, 2.07 or later (except 2.11.08) 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
@@ -518,8 +518,9 @@ Build Requirements
    launch a command prompt with the appropriate compiler paths automatically
    set.
 
--- NASM (http://www.nasm.us/) 0.98 or later (NASM 2.05 or later is required for
-   a 64-bit build)
+-- NASM (http://www.nasm.us) or YASM (http://yasm.tortall.net)
+   * If using NASM, 0.98 or later is required for an x86 build.
+   * If using NASM, 2.05 or later is required for an x86-64 build.
 
 -- If building the TurboJPEG Java wrapper, JDK 1.5 or later is required.  This
    can be downloaded from http://www.java.com.