stmhal, math: Define _M_LN2 if not already defined.

Addresses issue #790.

stmhal/math.c

@@ -102,6 +102,10 @@ float copysignf(float x, float y) {
 #ifdef log2f
 #undef log2f
 #endif
+// some compilers have _M_LN2 defined in math.h, some don't
+#ifndef _M_LN2
+#define _M_LN2 (0.69314718055994530942)
+#endif
 float log2f(float x) { return logf(x) / (float)_M_LN2; }
 
 static const float _M_LN10 = 2.30258509299404; // 0x40135d8e