build: teach apibuild.py to work in a non-srcdir build

* docs/Makefile.am (libvirt-api.xml libvirt-refs.xml): Generalize
apibuild.py to work in a non-srcdir build.  Pass "srcdir" to it.
* docs/apibuild.py (rebuild): Honor the $srcdir envvar.

docs/Makefile.am

@@ -132,7 +132,7 @@ devhelp/index.html devhelp/libvirt.devhelp: libvirt-api.xml $(devhelpxsl)
 libvirt-api.xml libvirt-refs.xml: apibuild.py \
 		$(srcdir)/../include/libvirt/*.h \
 		$(srcdir)/../src/libvirt.c $(srcdir)/../src/util/virterror.c
-	-$(srcdir)/apibuild.py
+	-srcdir=$(srcdir) $(srcdir)/apibuild.py
 
 clean-local:
 	rm -f *~ *.bak *.hierarchy *.signals *-unused.txt

docs/apibuild.py

@@ -2102,10 +2102,14 @@ class docBuilder:
 
 def rebuild():
     builder = None
-    if glob.glob("../src/libvirt.c") != [] :
+    srcdir = os.environ["srcdir"]
+    if glob.glob(srcdir + "/../src/libvirt.c") != [] :
         print "Rebuilding API description for libvirt"
-	builder = docBuilder("libvirt", ["../src", "../src/util", "../include/libvirt"],
-	                     [])
+	builder = docBuilder("libvirt",
+                             [srcdir + "/../src",
+                              srcdir + "/../src/util",
+                              srcdir + "/../include/libvirt"],
+                             [])
     elif glob.glob("src/libvirt.c") != [] :
         print "Rebuilding API description for libvir"
 	builder = docBuilder("libvirt", ["src", "src/util", "include/libvirt"],