Make examples friendlier for non GCC compilers and fixed slave_mem_i2c.c to compile on boards without I2C pins (#336)

spi/bme280_spi/bme280_spi.c

@@ -235,7 +235,5 @@ int main() {
 
         sleep_ms(1000);
     }
-
-    return 0;
 #endif
 }

spi/max7219_32x8_spi/max7219_32x8_spi.c

@@ -135,7 +135,5 @@ int main() {
 
         bright++;
     }
-
-    return 0;
 #endif
 }

spi/max7219_8x7seg_spi/max7219_8x7seg_spi.c

@@ -142,7 +142,5 @@ int main() {
             j = 0;
         }
     }
-
-    return 0;
 #endif
 }

spi/mpu9250_spi/mpu9250_spi.c

@@ -150,6 +150,4 @@ int main() {
 
         sleep_ms(100);
     }
-
-    return 0;
 }