pure C: fix Warnings

gcc provides good tips on pure C functions, let's ack't them.

pure: functions that are idempotent since they do not rely on internal
states. They are like some "const", weaker. They do not have side
effects.

use case:
  x = le16(p);
  y = le16(p);
could be optimized to:
  x = le16(p); # le16 is pure
  y = x;

Author: vjardin

src/pmbus_io.c

@@ -111,7 +111,7 @@ pmbus_lin16u_to_double(uint16_t raw, int exp5) {
 }
 
 uint16_t
-le16(const uint8_t *p) {
+BMR_PURE le16(const uint8_t *p) {
   return (uint16_t)(
         (uint16_t)p[0]
       | ((uint16_t)p[1] << 8)
@@ -120,7 +120,7 @@ le16(const uint8_t *p) {
 }
 
 uint32_t
-le32(const uint8_t *p) {
+BMR_PURE le32(const uint8_t *p) {
   return (uint32_t)  p[0]
        | ((uint32_t) p[1] << 8)
        | ((uint32_t) p[2] << 16)

src/pmbus_io.h

@@ -7,6 +7,12 @@
 
 #define D(xf) (double)((xf))
 
+#if defined(__GNUC__) || defined(__clang__)
+#  define BMR_PURE __attribute__((pure))
+#else
+#  define BMR_PURE
+#endif
+
 /* TODO: align with PMBus-Specification-Rev-1-3-1-Part-II-20150313.pdf */
 /*
  * PMBus standard (generic):