component/boost-lib: use simpler syntax for $PYTHON

The ${PYTHON:+--with-python=$PYTHON} looks like a buildout substitution and
because of the way slapos.recipe.cmmi replaces @@LOCATION@@ in options it was
not properly escaped when saved in .installed.cfg.
That was probably a bug, but we don't really need such a complex syntax,
because when using [python] section $PYTHON will be set in environment, so
instead of having the fallback, make sure python section was installed and
assume that $PYTHON is in environment.

component/boost-lib/buildout.cfg

@@ -6,6 +6,7 @@ extends =
   ../patch/buildout.cfg
   ../zlib/buildout.cfg
   ../xz-utils/buildout.cfg
+  ../defaults.cfg
 
 [boost-lib]
 recipe = slapos.recipe.cmmi
@@ -13,7 +14,7 @@ shared = true
 url = http://downloads.sourceforge.net/sourceforge/boost/boost_1_67_0.tar.bz2
 md5sum = ced776cb19428ab8488774e1415535ab
 location = @@LOCATION@@
-configure-command = ./bootstrap.sh --prefix=${:location} --without-icu $${PYTHON:+--with-python=$PYTHON}
+configure-command = ./bootstrap.sh --prefix=${:location} --without-icu --with-python=$PYTHON
 make-binary =
 make-options =
 make-targets = for a in $MAKEFLAGS; do case $a in -j*) j=$a; break;; esac; done;
@@ -29,3 +30,5 @@ patch-options = -p1
 patches =
   https://sources.debian.org/data/main/b/boost1.67/1.67.0-17/debian/patches/fix-ftbfs-python-3.3.patch#c85fb479d51354deafd1cc7af78f25d2
 patch-binary = ${patch:location}/bin/patch
+depends =
+  ${python:update}