gmp's -Wa,--noexecstack check is based on glibc's, so backport a change
that went into it there:
https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=215a87451d1fd623fd2699fa57f16f8ab6f6a2f3
https://sourceware.org/bugzilla/show_bug.cgi?id=6428

this is to workaround broken clang assembler that fails w/CFLAGS=-g:
http://crbug.com/585930
https://llvm.org/bugs/show_bug.cgi?id=11740

patch has been sent upstream too:
https://gmplib.org/list-archives/gmp-bugs/2016-February/003916.html

--- a/configure
+++ b/configure
@@ -11339,7 +11339,7 @@ EOF
   $as_echo "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
   test $ac_status = 0; }; } \
      && grep .note.GNU-stack conftest.s >/dev/null \
-     && { ac_try='${CC} $CFLAGS $CPPFLAGS -Wa,--noexecstack
+     && { ac_try='${CC} $ASMFLAGS -Wa,--noexecstack
                        -c -o conftest.o conftest.s >/dev/null'
   { { eval echo "\"\$as_me\":${as_lineno-$LINENO}: \"$ac_try\""; } >&5
   (eval $ac_try) 2>&5