waf: Disable build/install phase interleaving

When doing "./waf install" and some source files are modified
so they need to be rebuilt, the build and install phase can be
interleaved so one thread is still performing build while other
thread is already performing install tasks.

This appears to be a problem (at least on OS X) when some of
the Geany plugins are still being built and libgeany is already
being installed in parallel.

Create a separate group for the install phase to eliminate the
problem.

wscript

@@ -599,6 +599,9 @@ def build(bld):
                                'top_builddir': bld.out_dir,
                                'top_srcdir': bld.top_dir,})
 
+    # disable build/install phase interleaving
+    bld.add_group()
+
     ###
     # Install files
     ###