math.c: remove needless ifdef clause

* math.c (ifdef M_PI): M_PI is eventually defined in missing.h even if math.h
  doesn't supply it.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56810 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

math.c

@@ -989,12 +989,8 @@ InitVM_Math(void)
     rb_mMath = rb_define_module("Math");
     rb_eMathDomainError = rb_define_class_under(rb_mMath, "DomainError", rb_eStandardError);
 
-#ifdef M_PI
     /*  Definition of the mathematical constant PI as a Float number. */
     rb_define_const(rb_mMath, "PI", DBL2NUM(M_PI));
-#else
-    rb_define_const(rb_mMath, "PI", DBL2NUM(atan(1.0)*4.0));
-#endif
 
 #ifdef M_E
     /*  Definition of the mathematical constant E (e) as a Float number. */