Advise on use of int64_t cast

Advise on testing with clang compiler and using int64_t cast with PRIll
macro.

Fix GCC warning when 'make distcheck' is run.

README.developer

@@ -577,12 +577,13 @@ Hamlib should also compile with the following common compilers:
     * gcc-4.x and newer
     * in shared and static
     * C++ compiler against rig.h, riglist.h, rotator.h
+    * clang compiler
 
 Portability issues to watch:
 
     * little vs. big endian systems (use shifts or adhoc functions)
     * 64 bit int: avoid them in API
-    * printf/scanf of 64bit int: use PRIll and SCNll
+    * printf/scanf of 64bit int: use PRIll (cast value to int64_t) and SCNll
     * printf/scanf of freq_t: use PRIfreq and SCNfreq
 
 Testing:

tests/testbcd.c

@@ -40,7 +40,7 @@ int main (int argc, char *argv[])
 	printf("BCD: %2.2x",b[0]);
 	for (i = 1; i < (digits+1)/2; i++)
 		printf(",%2.2x",b[i]);
-	printf("\nResult after recoding: %"PRIll"\n", from_bcd(b, digits));
+	printf("\nResult after recoding: %"PRIll"\n", (int64_t)from_bcd(b, digits));
 
 	printf("\nBig Endian mode\n");
 	printf("Frequency: %"PRIfreq"\n",f);
@@ -48,7 +48,7 @@ int main (int argc, char *argv[])
 	printf("BCD: %2.2x",b[0]);
 	for (i = 1; i < (digits+1)/2; i++)
 		printf(",%2.2x",b[i]);
-	printf("\nResult after recoding: %"PRIll"\n", from_bcd_be(b, digits));
+	printf("\nResult after recoding: %"PRIll"\n", (int64_t)from_bcd_be(b, digits));
 
 	return 0;
 }