doc/Makefile.am: distribute Texinfo HTML and PDF

Build and distribute the HTML and PDF files generated from the Texinfo
source with 'make dist'.  Also install these files with 'make install'.

doc/Makefile.am

@@ -1,5 +1,7 @@
 EXTRA_DIST = hamlib.cfg index.doxygen hamlib.css footer.html
 
+dist_doc_DATA = hamlib.html hamlib.pdf
+
 SRCDOCLST = ../src/rig.c ../src/rotator.c ../src/tones.c ../src/locator.c \
 		../src/event.c ../src/conf.c ../src/mem.c ../src/settings.c