[LyX/master] Fix warnings in DTL targets on Windows (use STDC)

[Eugene Chornyi]

commit 6fac3144d585ad8bfa34c214e3a6db855ec99f68
Author: Eugene Chornyi <yu_jin at lyx.org>
Date:   Mon Sep 7 21:27:37 2020 +0200

    Fix warnings in DTL targets on Windows (use STDC)
---
 3rdparty/dtl/CMakeLists.txt |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/3rdparty/dtl/CMakeLists.txt b/3rdparty/dtl/CMakeLists.txt
index 999b8a2..f757b26 100644
--- a/3rdparty/dtl/CMakeLists.txt
+++ b/3rdparty/dtl/CMakeLists.txt
@@ -31,6 +31,9 @@ foreach(_t ${targets})
   add_executable(${_t} ${_t}.c)
   add_man(${_t}.man)
 endforeach()
+if (WIN32)
+  set_target_properties(${targets} PROPERTIES LANGUAGE C COMPILE_FLAGS -D__STDC__)
+endif()
 
 # Install dtl tools in this directory to omit possible packaging conflicts
 # if installing multiple lyx versions, each with own dtl executables