Fix nasm compile on OS X / Darwin

Apparently this had been fixes in Audacity and other projects
but no one had bothered to feed them back upstream. Thanks to
lvqcl for researching this and finding the relevant patches.
Thanks to Ozkan Sezer for validating them.

Closes:  https://sourceforge.net/p/flac/bugs/438/

src/libFLAC/ia32/nasm.h

@@ -46,12 +46,17 @@
 	%idefine code_section section .text
 	%idefine data_section section .data
 	%idefine bss_section  section .bss
+%elifdef OBJ_FORMAT_macho
+	%define FLAC__PUBLIC_NEEDS_UNDERSCORE
+	%idefine code_section section .text
+	%idefine data_section section .data
+	%idefine bss_section  section .bss
 %elifdef OBJ_FORMAT_elf
 	%idefine code_section section .text align=16
 	%idefine data_section section .data align=32
 	%idefine bss_section  section .bss  align=32
 %else
-	%error unsupported object format!
+	%error unsupported object format! ; this directive doesn't really work here
 %endif
 
 %imacro cglobal 1