Fix child-process communication (video monitor in particular)

103ef2ba08 introduced an API to write raw data (const void*)
to a child process, along with the previous API to
write (std::string const&)

VideoMonitor uses write_to_stdin("fixed text"), and g++
interprets this to use the (const void*) API instead
of the std::string, which breaks communication.

libs/pbd/pbd/system_exec.h

@@ -180,6 +180,14 @@ class LIBPBD_API SystemExec
 		 */
 		size_t write_to_stdin (std::string const& d, size_t len=0);
 
+		/** write into child-program's STDIN
+		 * @param d text to write
+		 * @param len length of data to write, if it is 0 (zero), d.length() is
+		 * used to determine the number of bytes to transmit.
+		 * @return number of bytes written.
+		 */
+		size_t write_to_stdin (const char* d, size_t len=0);
+
 		/** write into child-program's STDIN
 		 * @param data data to write
 		 * @param bytes length of data to write

libs/pbd/system_exec.cc

@@ -401,6 +401,15 @@ SystemExec::write_to_stdin (std::string const& d, size_t len)
 	return write_to_stdin ((const void*)data, len);
 }
 
+size_t
+SystemExec::write_to_stdin (const char* data, size_t len)
+{
+	if (len == 0) {
+		len = strlen (data);
+	}
+	return write_to_stdin ((const void*)data, len);
+}
+
 #ifdef PLATFORM_WINDOWS /* Windows Process */
 
 /* HELPER FUNCTIONS */