package/log4cxx: fix build without wchar

Fix the following build failure without wchar raised since bump to
version 0.12.0 in commit 83a0e8dea203dc37c6c207a36b3f46160578dcc2:

/home/buildroot/autobuild/instance-0/output-1/build/log4cxx-0.12.0/src/main/include/log4cxx/helpers/transcoder.h:113:33: error: 'wstring' in namespace 'std' does not name a type
  113 |   static void decode(const std::wstring& src, LogString& dst);
      |                                 ^~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/7d0297b071df06bc312b9927396021aa6fe401f4

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/log4cxx/log4cxx.mk

@@ -21,4 +21,10 @@ ifeq ($(BR2_PACKAGE_LIBESMTP),y)
 LOG4CXX_DEPENDENCIES += libesmtp
 endif
 
+ifeq ($(BR2_USE_WCHAR),y)
+LOG4CXX_CONF_OPTS += -DLOG4CXX_WCHAR_T=ON
+else
+LOG4CXX_CONF_OPTS += -DLOG4CXX_WCHAR_T=OFF
+endif
+
 $(eval $(cmake-package))