add the line 25-30 to make nullprpl protocol plugin can be compiled by the command `make nullprpl.so`

Makefile.am

EXTRA_DIST = \
	Makefile.mingw \
	README

pkgdir = $(libdir)/purple-$(PURPLE_MAJOR_VERSION)

NULLSOURCES = nullprpl.c

AM_CFLAGS = $(st)

libnull_la_LDFLAGS = -module -avoid-version

# nullprpl isn't built by default. when it is built, it's dynamically linked.
st =
pkg_LTLIBRARIES    = libnull.la
libnull_la_SOURCES = $(NULLSOURCES)
libnull_la_LIBADD  = $(GLIB_LIBS)

AM_CPPFLAGS = \
	-I$(top_srcdir)/libpurple \
	-I$(top_builddir)/libpurple \
	$(GLIB_CFLAGS) \
	$(DEBUG_CFLAGS)

SUFFIXES = .c .so
.c.so:
	$(LIBTOOL) --mode=compile $(CC) -DHAVE_CONFIG_H -I$(top_builddir) $(AM_CPPFLAGS) $(CFLAGS) -c $< -o tmp$@.lo $(PLUGIN_CFLAGS)
	$(LIBTOOL) --mode=link    $(CC) $(CFLAGS) -o libtmp$@.la -rpath $(pkgdir) tmp$@.lo $(LIBS) $(LDFLAGS) -module -avoid-version $(PLUGIN_LIBS)
	@rm -f tmp$@.lo tmp$@.o libtmp$@.la
	@cp .libs/libtmp$@*.so $@
	@rm -rf .libs/libtmp$@.*