boost: pass -dumpversion to user-config.jam

Short version avoids compiling problems with toolchains
containing long version string like Linaro toolchain.

[Peter: drop head/sed as -dumpversion returns version directly]
Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/boost/boost.mk

@@ -10,7 +10,7 @@ BOOST_SOURCE = boost_$(BOOST_FILE_VERSION).tar.bz2
 BOOST_SITE = http://$(BR2_SOURCEFORGE_MIRROR).dl.sourceforge.net/sourceforge/boost
 BOOST_INSTALL_STAGING = YES
 
-TARGET_CC_VERSION = $(shell $(TARGET_CC) --version | head -n 1 | sed -e "s/.*[[:space:]]\([[:digit:].]\+$$\)/\1/g" )
+TARGET_CC_VERSION = $(shell $(TARGET_CC) -dumpversion)
 
 BOOST_DEPENDENCIES = bzip2 zlib