toolchain: only fetch gcc/gmp/mpfr/mpc when using a buildroot toolchain

Closes #3103

gcc/gmp/mpfr/mpc are only needed when using an internal buildroot toolchain,
so only add them to HOST_SOURCE then.

Otherwise we get some nasty fetch error when doing "make source"

[Peter: add gmpc/mpfr/mpc and reword commit text]
Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
diff --git a/CHANGES b/CHANGES
index baddfad..a825511 100644
--- a/CHANGES
+++ b/CHANGES
@@ -44,6 +44,7 @@
 	#2965: Broken linkage to xkbcomp (blocking X server startup)
 	#2983: xlib_libX11 build failed
 	#3007: kexec doesn't build: Missing regdef.h file
+	#3103: make external-deps wants to download gcc-.tar.bz2 when...
 	#3109: abnormal `make busybox-menuconfig`
 
 2010.11, Released November 30th, 2010: