Better build support for custom baudrates

For some systems (OSes, versions, architectures) custom-baudrate
support is enabled by default.

For others, support may work, but must be enabled by defining
USE_CUSTOM_BAUD in the Makefile

You can also disable custom baudrate support altogether (even for
systems where it's enabled by default), by defining NO_CUSTOM_BAUD in
the Makefile

picocom.c

@@ -55,6 +55,8 @@
 #include "linenoise-1.0/linenoise.h"
 #endif
 
+#include "custbaud.h"
+
 /**********************************************************************/
 
 /* parity modes names */