Use Qt defined macros Q_BYTE_ORDER, Q_BIG_ENDIAN and
Q_LITLLE_ENDIAN which are defined also for musl libc.

--- ksplash/ksplashx/scale.cpp	2015-05-07 20:13:26.000000000 +0200
+++ ksplash/ksplashx/scale.cpp	2015-10-09 00:07:44.940604940 +0200
@@ -165,18 +165,18 @@
 
 // FIXME: replace with mRed, etc... These work on pointers to pixels, not
 // pixel values
-#if BYTE_ORDER == BIG_ENDIAN
+#if Q_BYTE_ORDER == Q_BIG_ENDIAN
 #define A_VAL(p) ((unsigned char *)(p))[0]
 #define R_VAL(p) ((unsigned char *)(p))[1]
 #define G_VAL(p) ((unsigned char *)(p))[2]
 #define B_VAL(p) ((unsigned char *)(p))[3]
-#elif BYTE_ORDER == LITTLE_ENDIAN
+#elif Q_BYTE_ORDER == Q_LITTLE_ENDIAN
 #define A_VAL(p) ((unsigned char *)(p))[3]
 #define R_VAL(p) ((unsigned char *)(p))[2]
 #define G_VAL(p) ((unsigned char *)(p))[1]
 #define B_VAL(p) ((unsigned char *)(p))[0]
 #else
-#error "BYTE_ORDER is not defined"
+#error "Q_BYTE_ORDER is not defined"
 #endif
 
 #define INV_XAP                   (256 - xapoints[x])