Generic configury support for shared libs on VxWorks

This change adds the configury bits to activate the build of
shared libs on VxWorks ports configured with --enable-shared,
for libraries variants where this is generally supported (rtp,
code model !large - currently not compatible with -fPIC).

Set lt_cv_deplibs_check_method in libtool.m4, so the build of
libraries know how to establish dependencies.  This is useful in
configurations such as aarch64 where proper support of LSE relies
on accurate dependency information between libstdc++ and libgcc_s
to begin with.

	* libtool.m4 (*vxworks*): When enable_shared, set dynamic_linker
	and friends for rtp !large. Assume the linker has the required
	abilities and set lt_cv_deplibs_check_method.

libtool.m4

@@ -2700,6 +2700,25 @@ uts4*)
   shlibpath_var=LD_LIBRARY_PATH
   ;;
 
+# Shared libraries for VwWorks, >= 7 only at this stage
+# and (fpic) still incompatible with "large" code models
+# in a few configurations. Only for RTP mode in any case,
+# and upon explicit request at configure time.
+vxworks7*)
+  dynamic_linker=no
+  case ${with_multisubdir}-${enable_shared} in
+    *large*)
+      ;;
+    *mrtp*-yes)
+      version_type=linux
+      need_lib_prefix=no
+      need_version=no
+      library_names_spec='${libname}${release}${shared_ext}$versuffix ${libname}${release}${shared_ext}$major $libname${shared_ext}'
+      soname_spec='${libname}${release}${shared_ext}$major'
+      dynamic_linker="$host_os module_loader"
+      ;;
+  esac
+  ;;
 *)
   dynamic_linker=no
   ;;
@@ -3187,6 +3206,11 @@ sysv4 | sysv4.3*)
 tpf*)
   lt_cv_deplibs_check_method=pass_all
   ;;
+vxworks*)
+  # Assume VxWorks cross toolchains are built on Linux, possibly
+  # as canadian for Windows hosts.
+  lt_cv_deplibs_check_method=pass_all
+  ;;
 esac
 ])
 file_magic_cmd=$lt_cv_file_magic_cmd
@@ -6429,8 +6453,9 @@ if test "$_lt_caught_CXX_error" != yes; then
         ;;
 
       vxworks*)
-        # FIXME: insert proper C++ library support
-        _LT_TAGVAR(ld_shlibs, $1)=no
+        # For VxWorks ports, we assume the use of a GNU linker with
+        # standard elf conventions.
+        _LT_TAGVAR(ld_shlibs, $1)=yes
         ;;
 
       *)