update compatibility mode for different F_CPU values

libraries/SPI/SPI.h

@@ -145,14 +145,13 @@ extern SPIClass SPI1;
 
 // For compatibility with sketches designed for AVR @ 16 MHz
 // New programs should use SPI.beginTransaction to set the SPI clock
-#if F_CPU == 16000000
-  #define SPI_CLOCK_DIV2   2
-  #define SPI_CLOCK_DIV4   4
-  #define SPI_CLOCK_DIV8   8
-  #define SPI_CLOCK_DIV16  16
-  #define SPI_CLOCK_DIV32  32
-  #define SPI_CLOCK_DIV64  64
-  #define SPI_CLOCK_DIV128 128
-#endif
-
+#ifdef F_CPU
+#define DIVISOR F_CPU / 16000000
+#define SPI_CLOCK_DIV2   2 * DIVISOR
+#define SPI_CLOCK_DIV4   4 * DIVISOR
+#define SPI_CLOCK_DIV8   8 * DIVISOR
+#define SPI_CLOCK_DIV16  16 * DIVISOR
+#define SPI_CLOCK_DIV32  32 * DIVISOR
+#define SPI_CLOCK_DIV64  64 * DIVISOR
+#define SPI_CLOCK_DIV128 128 * DIVISOR
 #endif