This patch is an improvement on the one posted here:
  Bug 528200 - media-libs/x265 - /usr/lib64/libx265.a has executable stack
  https://bugs.gentoo.org/528200

--- source/common/x86/x86inc.asm.orig	2017-11-28 22:21:40.000000000 -0500
+++ source/common/x86/x86inc.asm	2018-02-25 16:36:54.243966522 -0500
@@ -66,6 +66,15 @@
     %endif
 %endif
 
+%define FORMAT_ELF 0
+%ifidn __OUTPUT_FORMAT__,elf
+    %define FORMAT_ELF 1
+%elifidn __OUTPUT_FORMAT__,elf32
+    %define FORMAT_ELF 1
+%elifidn __OUTPUT_FORMAT__,elf64
+    %define FORMAT_ELF 1
+%endif
+
 %ifdef PREFIX
     %define mangle(x) _ %+ x
 %else
@@ -685,7 +694,7 @@
         CAT_XDEFINE cglobaled_, %2, 1
     %endif
     %xdefine current_function %2
-    %ifidn __OUTPUT_FORMAT__,elf
+    %if FORMAT_ELF
         global %2:function %%VISIBILITY
     %else
         global %2
@@ -718,7 +727,7 @@
 
 %macro const 1-2+
     %xdefine %1 mangle(private_prefix %+ _ %+ %1)
-    %ifidn __OUTPUT_FORMAT__,elf
+    %if FORMAT_ELF
         global %1:data hidden
     %else
         global %1
@@ -729,7 +738,7 @@
 
 ; This is needed for ELF, otherwise the GNU linker assumes the stack is
 ; executable by default.
-%ifidn __OUTPUT_FORMAT__,elf
+%if FORMAT_ELF
     [SECTION .note.GNU-stack noalloc noexec nowrite progbits]
 %endif