Fix doc generation in makefile rules
diff --git a/Makefile.am b/Makefile.am
index 56e70bb..fb77f80 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -7,12 +7,14 @@
 pkgconfigdir = $(libdir)/pkgconfig
 pkgconfig_DATA = libimobiledevice-1.0.pc
 
-EXTRA_DIST= docs
+EXTRA_DIST = docs
 
-docs:
-	rm -fr docs
+docs/html:
+	rm -rf docs/html
 	doxygen doxygen.cfg
 
+docs: doxygen.cfg docs/html
+
 indent:
 	indent -kr -ut -ts4 -l120 src/*.c src/*.h dev/*.c