Create includedir on call to install-exec-hook

Debian maintainers reported that liberasurecode could
not be built reproducibly.

What happens is that erasurecode_version.h headers are
non-determinstically installed in the target directory
depending on the system clock. This is due to
debian/tmp/usr/include not being created and the
install-exec-hook ignores errors.

The attached patch ensures target ${includedir} exists
and therefore the headers will always be there.

Signed-off-by: Chris Lamb <lamby@debian.org>

Makefile.am

@@ -32,7 +32,8 @@ thisinclude_HEADERS = \
 	include/rs_vand/liberasurecode_rs_vand.h
 
 install-exec-hook:
-	-(cd $(DESTDIR)$(includedir) && \
+	-(mkdir -p $(DESTDIR)$(includedir) && \
+	   cd $(DESTDIR)$(includedir) && \
 	   rm -f erasurecode.h erasurecode_version.h \
 	         erasurecode_stdinc.h erasurecode_helpers.h \
 	         config_liberasurecode.h && \