toolchain/external: only copy the pthread lib if needed

If threads are disabled, do not try to copy the libpthread.so from the
external library.

It is still expected that the BR configuration matches the external
toolchain setup, and no check is done to enforce that.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

toolchain/external-toolchain/ext-tool.mk

@@ -251,7 +251,7 @@ check_cross_compiler_exists = \
 
 uclibc: dependencies $(STAMP_DIR)/ext-toolchain-installed
 
-EXTERNAL_LIBS=libc.so libcrypt.so libdl.so libgcc_s.so libm.so libnsl.so libpthread.so libresolv.so librt.so libutil.so
+EXTERNAL_LIBS=libc.so libcrypt.so libdl.so libgcc_s.so libm.so libnsl.so libresolv.so librt.so libutil.so
 ifeq ($(BR2_TOOLCHAIN_EXTERNAL_UCLIBC),y)
 EXTERNAL_LIBS+=ld-uClibc.so
 else
@@ -262,6 +262,10 @@ ifeq ($(BR2_INSTALL_LIBSTDCPP),y)
 EXTERNAL_LIBS+=libstdc++.so
 endif
 
+ifneq ($(BR2_PTHREADS_NONE),y)
+EXTERNAL_LIBS+=libpthread.so
+endif # ! no threads
+
 # SYSROOT_DIR selection. We first try the -print-sysroot option,
 # available in gcc 4.4.x and in some Codesourcery toolchains. If this
 # option is not available, we fallback to the value of --with-sysroot