Fix python build dependencies

Also pass down BOOST_ROOT and PYTHON_VERSION variables.

Makefile

@@ -122,6 +122,12 @@ PVAPY_CONFIG += EPICS4_DIR=$(abspath .)
 ifneq ($(wildcard $(BOOST_NUMPY)),)
   PVAPY_CONFIG += BOOST_NUM_PY_DIR=$(BOOST_NUMPY)
 endif
+ifneq ($(wildcard $(BOOST_ROOT)),)
+  PVAPY_CONFIG += BOOST_ROOT=$(BOOST_ROOT)
+endif
+ifneq ($(PYTHON_VERSION),)
+  PVAPY_CONFIG += PYTHON_VERSION=$(PYTHON_VERSION)
+endif
 
 ifeq ($(filter sphinx,$(MAKECMDGOALS)),sphinx)
   PYTHON_VER = $(shell python -c 'import sys; print sys.version[:3]')
@@ -137,7 +143,7 @@ endif
 pvaPy: config.pvaPy host.pvaPy
 config.pvaPy: pvaPy/configure/RELEASE.local \
     host.pvaClientCPP host.pvDatabaseCPP
-pvaPy/configure/RELEASE.local:
+pvaPy/configure/RELEASE.local: | host.pvaClientCPP host.pvDatabaseCPP
 	$(MAKE) -C pvaPy configure $(PVAPY_CONFIG)
 host.pvaPy: pvaPy/configure/RELEASE.local
 	$(MAKE) -C pvaPy $(EPICS_HOST_ARCH)

README.md

@@ -63,6 +63,10 @@ This bundle includes a copy of the pvaPy python module, but the build commands a
 
     make python
 
+or for Python 3:
+
+    make python PYTHON_VERSION=3
+
 If you only want to build EPICS V4 for the python API, you can avoid compiling some of the modules in the first step above by just running
 
     make EPICS_BASE=/path/to/epics/base python
@@ -75,6 +79,8 @@ or
 
     make EPICS_BASE=/path/to/epics/base BOOST_NUMPY=/path/to/boost/numpy python
 
+The top-level build system understands the variable BOOST_ROOT if you need to configure pyaPy to build against a custom Boost installation; see the pvaPy/README.md file for more information about using this.
+
 After building pvaPy you can also generate the related reference documentation if you have the Sphinx Python documentation generator installed:
 
     make sphinx