fixup! boost-lib: enable parallel build

This fixes commit f9d5bde3.

We also often have MAKEFLAGS with -l option. For b2, this option sets
a time limit in seconds for each job, which led to random failures.

component/boost-lib/buildout.cfg

@@ -13,9 +13,10 @@ url = http://downloads.sourceforge.net/sourceforge/boost/boost_1_67_0.tar.bz2
 md5sum = ced776cb19428ab8488774e1415535ab
 location = @@LOCATION@@
 configure-command = ./bootstrap.sh --prefix=${:location} --with-python=${python2.7:location}/bin/python2.7
-make-binary = ./b2
-# XXX: we assume that $MAKEFLAGS is only used for -j
-make-options = $MAKEFLAGS link=shared dll-path=${:location}/lib:${bzip2:location}/lib:${gcc:location}/lib:${gcc:location}/lib64:${zlib:location}/lib
+make-binary =
+make-options =
+make-targets = for a in $MAKEFLAGS; do case $a in -j*) j=$a; break;; esac; done;
+  ./b2 $j link=shared dll-path=${:location}/lib:${bzip2:location}/lib:${gcc:location}/lib:${gcc:location}/lib64:${zlib:location}/lib install
 environment =
   PATH=${gcc:location}/bin:%(PATH)s
   BZIP2_INCLUDE=${bzip2:location}/include