Add a common makefile named config.mk for overall configuration parameters

Add a common makefile named config.mk for overall configuration parameters and a future location for OS-specific customization.  Currently only DEBUG is defined.  If set, this flag compiles with debugging enabled by default.  You can also do 'make DEBUG=1' from the command line

Also, for linux c.cc change:
// ***** ADDED BY AREINER FROM GOATPIG'S WINDOWS-PORT INSTRUCTIONS
 #define strdup _strdup
 // ***** ADDED BY AREINER FROM GOATPIG'S WINDOWS-PORT INSTRUCTIONS
 
needs:
#if defined(_WIN64) | defined(_WIN32)
#define strdup _strdup
#endif