https://bugs.gentoo.org/548310

apply boost.m4 updates to fix building w/gcc-5

--- a/configure
+++ b/configure
@@ -17481,8 +17481,10 @@
 boost-lib-version = BOOST_LIB_VERSION
 _ACEOF
 if (eval "$ac_cpp conftest.$ac_ext") 2>&5 |
+  grep -v '#' |
   tr -d '\r' |
-  $SED -n -e "/^boost-lib-version = /{s///;s/\"//g;p;q;}" >conftest.i 2>&1; then :
+  tr -s '\n' ' ' |
+  $SED -n -e "/^boost-lib-version = /{s///;s/[\" ]//g;p;q;}" >conftest.i 2>&1; then :
   boost_cv_lib_version=`cat conftest.i`
 fi
 rm -rf conftest*