editors/libreoffice: fix build with lld 17

Building editors/libreoffice with lld 17 results in the following link
errors:

    ld: error: version script assignment of 'PRIVATE_1.4' to symbol '_ZN3sal13backtrace_getEm' failed: symbol not defined
    ld: error: version script assignment of 'GLIBCXX_3.4' to symbol '_ZNSs4_Rep20_S_empty_rep_storageE' failed: symbol not defined

Upstream fixed this only for Linux with:
LibreOffice/core@baddc74

Add the same block to the more generic unxgcc.mk makefile, so it gets
picked up for FreeBSD too.

PR:	274697
Author:    Dimitry Andric <dim@FreeBSD.org>

editors/libreoffice/files/patch-solenv_gbuild_platform_unxgcc.mk

@@ -1,6 +1,17 @@
---- solenv/gbuild/platform/unxgcc.mk.orig	2022-12-22 00:14:09 UTC
+--- solenv/gbuild/platform/unxgcc.mk.orig	2023-09-22 19:07:01 UTC
 +++ solenv/gbuild/platform/unxgcc.mk
-@@ -55,8 +55,7 @@ gb_CXXFLAGS := \
+@@ -17,6 +17,10 @@
+ #   the License at http://www.apache.org/licenses/LICENSE-2.0 .
+ #
+
++ifneq ($(findstring lld,$(USE_LD)),)
++USE_LD += -Wl,--undefined-version
++endif
++
+ # to block heavy exception handling that try to acquire the solarmutex
+ export LO_LEAN_EXCEPTION=1
+
+@@ -55,8 +59,7 @@ gb_CXXFLAGS := \
  gb_CXXFLAGS := \
  	$(gb_CXXFLAGS_COMMON) \
  	-fPIC \