Ship the headers in a directory

So that we can eventually rename the directory to have a Waltham 2.0,
if we ever have to break the API.

Users of the library should #include <waltham-connection.h> rather
than <waltham/waltham-connection.h>, and so on.

Signed-off-by: Emilio Pozuelo Monfort <emilio.pozuelo@collabora.co.uk>

data/waltham.pc.in

@@ -6,5 +6,5 @@ includedir=@includedir@
 Name: Waltham
 Description: Waltham library
 Version: @VERSION@
-Cflags: -I${includedir}
+Cflags: -I${includedir}/waltham
 Libs: -L${libdir} -lwaltham

src/waltham/Makefile.am

@@ -98,12 +98,13 @@ nodist_libwaltham_la_SOURCES = \
 	server-serialice.c \
 	server-deserialice.c
 
-include_HEADERS = \
+waltham_includedir = $(includedir)/waltham
+waltham_include_HEADERS = \
 	waltham-connection.h \
 	waltham-object.h \
 	waltham-util.h \
 	$(NULL)
 
-nodist_include_HEADERS = \
+nodist_waltham_include_HEADERS = \
 	waltham-client.h \
 	waltham-server.h