gcc-5.2: disable isl

We presently don't package isl. Unfortunately, if the host is already
using gcc-5.2 (as is presently the case on Arch Linux), configure will
autodetect the host's libisl, and do_compile will break because the
system isl headers aren't pulled in. In lieu of packaging isl, disable
it for now.

[YOCTO #8376]

(From OE-Core rev: 555e8d110435cf4af1e1ab4699c2fa55898e9d80)

Signed-off-by: Richard Tollerton <rich.tollerton@ni.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/gcc/gcc-5.2.inc

@@ -98,6 +98,7 @@ EXTRA_OECONF_BASE = "\
     --with-cloog=no \
     --enable-checking=release \
     --enable-cheaders=c_global \
+    --without-isl \
 "
 
 EXTRA_OECONF_INITIAL = "\
@@ -109,6 +110,7 @@ EXTRA_OECONF_INITIAL = "\
     --disable-lto \
     --disable-plugin \
     --enable-decimal-float=no \
+    --without-isl \
 "
 
 EXTRA_OECONF_append_libc-uclibc = " --disable-decimal-float "