doc/Makefile.am: Don't build/distribute PDF manual

Building PDF may be problematic with older texinfo tools so don't build
or distribute the PDF version of the manual.  On systems with a newer
texinfo installation, 'make pdf' will generate a print version of the
manual.

doc/Makefile.am

@@ -1,6 +1,6 @@
 EXTRA_DIST = hamlib.cfg index.doxygen hamlib.css footer.html
 
-dist_doc_DATA = Hamlib_design.png hamlib.html hamlib.pdf
+dist_doc_DATA = Hamlib_design.png hamlib.html
 
 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