lib/libm: Define _IEEE_LIBM only if not set.

fdilibm was originally meant to see _IEEE_LIBM defined from outside the
libm code, not it being hardcoded in.  Picolibc assumes this assumption
holds true and attempts to define itself, conflicting with the existing
definition.

Signed-off-by: Alessandro Gatti <[email protected]>

Author: agatti

lib/libm/wf_lgamma.c

@@ -24,7 +24,9 @@
  */
 
 #include "fdlibm.h"
+#ifndef _IEEE_LIBM
 #define _IEEE_LIBM 1
+#endif
 
 #ifdef __STDC__
 	float lgammaf(float x)

lib/libm/wf_tgamma.c

@@ -24,7 +24,9 @@
 
 #include "math.h"
 #include "fdlibm.h"
+#ifndef _IEEE_LIBM
 #define _IEEE_LIBM 1
+#endif
 
 #ifdef __STDC__
 	float tgammaf(float x)