Work around issue with isl not downloading for Windows and Linux builds of crosstool-ng.

Makefile

@@ -25,9 +25,12 @@ $(crosstool-ng):
 toolchain-lin := $(LOCAL_DIR)/x86_64-ubuntu16.04-linux-gnu
 toolchain-lin: $(toolchain-lin)
 $(toolchain-lin): $(crosstool-ng)
+	# HACK until crosstool-ng has fixed its mirror for isl library
+	-mkdir /home/build/src
+	cd /home/build/src && wget ftp.halifax.rwth-aachen.de/gentoo/distfiles/isl-0.24.tar.xz
 	ct-ng x86_64-ubuntu16.04-linux-gnu
 	CT_PREFIX="$(LOCAL_DIR)" ct-ng build
-	rm -rf .build .config build.log
+	rm -rf .build .config build.log /home/build/src
 	# HACK Copy GL include dir to toolchain sysroot
 	chmod +w $(toolchain-lin)/x86_64-ubuntu16.04-linux-gnu/sysroot/usr/include
 	cp -r /usr/include/GL $(toolchain-lin)/x86_64-ubuntu16.04-linux-gnu/sysroot/usr/include/
@@ -37,9 +40,12 @@ $(toolchain-lin): $(crosstool-ng)
 toolchain-win := $(LOCAL_DIR)/x86_64-w64-mingw32
 toolchain-win: $(toolchain-win)
 $(toolchain-win): $(crosstool-ng)
+	# HACK until crosstool-ng has fixed its mirror for isl library
+	-mkdir /home/build/src
+	cd /home/build/src && wget ftp.halifax.rwth-aachen.de/gentoo/distfiles/isl-0.24.tar.xz
 	ct-ng x86_64-w64-mingw32
 	CT_PREFIX="$(LOCAL_DIR)" ct-ng build
-	rm -rf .build .config build.log
+	rm -rf .build .config build.log /home/build/src
 
 
 toolchain-mac := $(LOCAL_DIR)/osxcross