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>

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:

toolchain/gcc/gcc-uclibc-4.x.mk

@@ -109,12 +109,16 @@ endif
 
 GCC_WITH_HOST_GMP = --with-gmp=$(HOST_DIR)/usr
 GCC_WITH_HOST_MPFR = --with-mpfr=$(HOST_DIR)/usr
+ifeq ($(BR2_TOOLCHAIN_BUILDROOT),y)
 HOST_SOURCE += host-gmp-source host-mpfr-source
+endif
 GCC_HOST_PREREQ = host-gmp host-mpfr
 
 ifeq ($(findstring x4.5.,x$(GCC_VERSION)),x4.5.)
 GCC_WITH_HOST_MPC = --with-mpc=$(HOST_DIR)/usr
+ifeq ($(BR2_TOOLCHAIN_BUILDROOT),y)
 HOST_SOURCE += host-mpc-source
+endif
 GCC_HOST_PREREQ += host-mpc
 endif
 
@@ -152,7 +156,9 @@ GCC_CONF_ENV:=acx_cv_prog_cc_pedantic__Wno_long_long__Wno_variadic_macros_______
 	acx_cv_prog_cc_warning__Wno_overlength_strings=no
 endif
 
+ifeq ($(BR2_TOOLCHAIN_BUILDROOT),y)
 HOST_SOURCE+=gcc-source
+endif
 
 $(DL_DIR)/$(GCC_SOURCE):
 	mkdir -p $(DL_DIR)