cmake: eliminate a compile-time warning

In file included from /usr/include/boost/bind.hpp:30,
                 from /usr/include/boost/property_tree/json_parser/detail/parser.hpp:7,
                 from /usr/include/boost/property_tree/json_parser/detail/read.hpp:13,
                 from /usr/include/boost/property_tree/json_parser.hpp:16,
                 from /home/kdudka/git/csdiff/src/json-writer.cc:29:
/usr/include/boost/bind.hpp:36:1: note: ‘#pragma message: The practice of declaring the Bind placeholders (_1, _2, ...) in the global namespace is deprecated. Please use <boost/bind/bind.hpp> + using namespace boost::placeholders, or define BOOST_BIND_GLOBAL_PLACEHOLDERS to retain the current behavior.’
   36 | BOOST_PRAGMA_MESSAGE(
      | ^~~~~~~~~~~~~~~~~~~~

In file included from /usr/include/boost/bind.hpp:30,
                 from /usr/include/boost/property_tree/json_parser/detail/parser.hpp:7,
                 from /usr/include/boost/property_tree/json_parser/detail/read.hpp:13,
                 from /usr/include/boost/property_tree/json_parser.hpp:16,
                 from /home/kdudka/git/csdiff/src/json-parser.cc:27:
/usr/include/boost/bind.hpp:36:1: note: ‘#pragma message: The practice of declaring the Bind placeholders (_1, _2, ...) in the global namespace is deprecated. Please use <boost/bind/bind.hpp> + using namespace boost::placeholders, or define BOOST_BIND_GLOBAL_PLACEHOLDERS to retain the current behavior.’
   36 | BOOST_PRAGMA_MESSAGE(
      | ^~~~~~~~~~~~~~~~~~~~

This is a follow-up to csdiff-2.1.0-12-g84511fd, which did not fix
the problem completely.

Author: kdudka

src/CMakeLists.txt

@@ -17,6 +17,14 @@
 
 add_definitions(-Wall -Wextra -fPIC -std=c++11)
 
+# eliminate the following warning:
+#   /usr/include/boost/bind.hpp:36:1: note: ‘#pragma message: The practice
+#   of declaring the Bind placeholders (_1, _2, ...) in the global namespace
+#   is deprecated. Please use <boost/bind/bind.hpp> + using namespace
+#   boost::placeholders, or define BOOST_BIND_GLOBAL_PLACEHOLDERS to retain
+#   the current behavior.’
+add_definitions(-DBOOST_BIND_GLOBAL_PLACEHOLDERS)
+
 # cslib.a
 add_library(cs STATIC
     abstract-filter.cc

src/pycsdiff.cc

@@ -23,14 +23,6 @@
 
 #include <sstream>
 
-// eliminate the following warning:
-//   /usr/include/boost/bind.hpp:36:1: note: ‘#pragma message: The practice
-//   of declaring the Bind placeholders (_1, _2, ...) in the global namespace
-//   is deprecated. Please use <boost/bind/bind.hpp> + using namespace
-//   boost::placeholders, or define BOOST_BIND_GLOBAL_PLACEHOLDERS to retain
-//   the current behavior.’
-#define BOOST_BIND_GLOBAL_PLACEHOLDERS
-
 #include <boost/python.hpp>
 
 std::string diff_scans(