[source] toolchain/gcc: fix --with-isl path

[LEDE Commits]

nbd pushed a commit to source.git, branch master:
https://git.lede-project.org/c87aa0d7ca8f2dcd3cf559a4933fef3cb44f7a21

commit c87aa0d7ca8f2dcd3cf559a4933fef3cb44f7a21
Author: Syrone Wong <wong.syrone at gmail.com>
AuthorDate: Tue May 30 15:10:02 2017 +0800

    toolchain/gcc: fix --with-isl path
    
    This fixes GCC 7 compilation when GRAPHITE is selected.
    The path is replaced with $(TOOLCHAIN_DIR), while it should be
    $(STAGING_DIR_HOST). To keep in sync with the path of gmp,
    mpfr and mpc, I'm using $(TOPDIR)/staging_dir/host.
    
    Fixes: f62f4b3c5c9d ("build: stop overriding STAGING_DIR_HOST for toolchain build")
    
    Signed-off-by: Syrone Wong <wong.syrone at gmail.com>
---
 toolchain/gcc/common.mk | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/toolchain/gcc/common.mk b/toolchain/gcc/common.mk
index 55ae1a2..b28d3ce 100644
--- a/toolchain/gcc/common.mk
+++ b/toolchain/gcc/common.mk
@@ -91,9 +91,9 @@ ifdef CONFIG_INSTALL_GCCGO
 endif
 
 ifdef CONFIG_GCC_USE_GRAPHITE
-  GRAPHITE_CONFIGURE=--with-isl=$(HOST_BUILD_PREFIX)
+  GRAPHITE_CONFIGURE:= --with-isl=$(TOPDIR)/staging_dir/host
 else
-  GRAPHITE_CONFIGURE=--without-isl --without-cloog
+  GRAPHITE_CONFIGURE:= --without-isl --without-cloog
 endif
 
 GCC_CONFIGURE:= \