[LyX/master] Do not compile trivstring.cpp if not needed

[Georg Baum]

commit 2e5ecd9dfbd6a9068ba384bf60726d5eae546b84
Author: Georg Baum <b...@lyx.org>
Date:   Fri Dec 26 20:09:58 2014 +0100

    Do not compile trivstring.cpp if not needed
    
    This avoids a MSVC warning.

diff --git a/src/support/CMakeLists.txt b/src/support/CMakeLists.txt
index b0d1c6d..42a9134 100644
--- a/src/support/CMakeLists.txt
+++ b/src/support/CMakeLists.txt
@@ -33,6 +33,9 @@ list(REMOVE_ITEM support_sources
        ${TOP_SRC_DIR}/src/support/os_cygwin.cpp
        ${TOP_SRC_DIR}/src/support/os_os2.C
        ${TOP_SRC_DIR}/src/support/gettext.cpp)
+if(NOT STD_STRING_USES_COW)
+       list(REMOVE_ITEM support_sources 
${TOP_SRC_DIR}/src/support/trivstring.cpp)
+endif()
 
 if(APPLE)
        message(STATUS "Mac LinkBack support")