(libnettle_a_SOURCES): List only .c files. Headers

moved to noinst_HEADERS.
(SHLIBOBJECTS): Substitute from libnettle_a_SOURCES, not
am_libnettle_a_OBJECTS, since the latter includes
libnettle_a-prefixes with some automake versions.
(SHLIBSONAME): Check if this name is empty, which is the case on
cygwin, before using it.

Rev: src/nettle/Makefile.am:1.79