update build script

Author: dead-beef

Diff for: build.sh

@@ -1,5 +1,9 @@
 #!/bin/sh
 
+if [ -d env ]; then
+    . env/bin/activate
+fi
+
 rm -rfv build/* dist/* docs/* docsrc/source/* \
     && make -C docsrc apidoc html \
     && python setup.py sdist bdist_wheel