cc-tool: fix static build problem with boost regex

The boost.m4 logic is a bit crappy, and needs some help to know that
boost_regex needs the pthread library.

Fixes:

  http://autobuild.buildroot.org/results/69a/69aff0b9d0ac8fe08e1d2f7ffb691f6a8fc8693b/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/cc-tool/cc-tool.mk

@@ -14,4 +14,10 @@ CC_TOOL_DEPENDENCIES = boost libusb
 # Configure script "discovers" boost in /usr/local if not given explicitly
 CC_TOOL_CONF_OPTS = --with-boost=$(STAGING_DIR)/usr
 
+# Help boost.m4 find the Boost Regex library, which needs the pthread
+# library, but isn't detected using a modern (pkg-config) mechanism.
+ifeq ($(BR2_STATIC_LIBS),y)
+CC_TOOL_CONF_ENV += LIBS="-lpthread"
+endif
+
 $(eval $(autotools-package))