Makefile.in -- minimize changes to original file

Minimize the changes in the top level Makefile.in wrt. github/master branch to allow for easy migration / pull-in of github/develop branch.

• remove the added header that provides info on copyright for changes and pointer to original license.
• revert the change that now creates a libcoap_phoenix library. Just keep the original name libcoap (see also notes in discussion below).
• bring back $(MAKE) -C examples check (delete comment sign)

Notes:

• We changed the name of the library to make it very transparent that the libcoap we produce is not compatible with the original libcoap. @aon -- eventually, we need to decide if we want to keep this name change (to avoid in the future confusion if for any reason two versions of libcoap are found on a system) or if we just want to leave the original name.
• $(MAKE) -C examples check is likely commented out as the modifications to libcoap did not allow to run the examples any more. If the build fails for that, comment out $(MAKE) -C examples check again.
• we kept the modification in (reasons per comments below)

$(LIBSO):	$(OBJECTS)
	$(CC) $(LDSOFLAGS) $(LDFLAGS) -o $@ $^
	# $(LD) $(LDSOFLAGS) $(LDFLAGS) -o $@ $^
	# It is not recommended to use ld for creating dynamic libraries
	# see: https://gcc.gnu.org/faq.html#picflag-needed
	# Hence, we changed $(LD) to $(CC) as we run into problems
	# when compiling the unmodified, standard libcoap code on
	# Rhaspberry Pi.