Make libvnc.so loadable: link libcore and the external libraries it needs.

Two faults in tigervnc 1.16.2's unix/xserver/hw/vnc/Makefile.am, both of which
only bite the loadable Xorg module; Xvnc itself is fine because it links via
LOCAL_LIBS and XSERVER_SYS_LIBS.

1. CORE_LIB is used in COMMON_LIBS and never defined.  1.16 renamed the common
   library to "core", added $(CORE_LIB) to the list and forgot the definition,
   so it expands to nothing and common/core/libcore.la is never linked.
   Result: 96 undefined core:: symbols and
     (EE) Failed to load .../libvnc.so: undefined symbol: _ZTIN4core13VoidParameterE

2. The module never links the external libraries those static archives depend
   on.  libvnc.la is built with -Wl,-z,now, so every symbol must resolve at
   dlopen time; pixman and zlib come from the Xorg server, but gnutls, nettle,
   hogweed, libjpeg and PAM do not.
   Result, once (1) is fixed:
     (EE) Failed to load .../libvnc.so: undefined symbol: gnutls_alert_get

Found on Parted Magic 2026-09-23 building 1.16.2 against xorg-server 21.1.24.

--- a/unix/xserver/hw/vnc/Makefile.am
+++ b/unix/xserver/hw/vnc/Makefile.am
@@ -2,6 +2,7 @@
 TIGERVNC_BUILDDIR=$(abspath ${top_builddir}/../..)
 
 RFB_LIB=$(TIGERVNC_BUILDDIR)/common/rfb/librfb.la
+CORE_LIB=$(TIGERVNC_BUILDDIR)/common/core/libcore.la
 RDR_LIB=$(TIGERVNC_BUILDDIR)/common/rdr/librdr.la
 OS_LIB=$(TIGERVNC_BUILDDIR)/common/os/libos.la
 NETWORK_LIB=$(TIGERVNC_BUILDDIR)/common/network/libnetwork.la
@@ -84,6 +85,6 @@
 
 libvnc_la_LDFLAGS = -module -avoid-version -Wl,-z,now
 
-libvnc_la_LIBADD = libvnccommon.la $(COMMON_LIBS)
+libvnc_la_LIBADD = libvnccommon.la $(COMMON_LIBS) -lgnutls -lnettle -lhogweed -ljpeg -lpam
 
 EXTRA_DIST = Xvnc.man