trio.c: Define modl, floorl and powl only when needed

They may have been defined on Visual Studio before 2013, so check for
their presence first.

trio.c

@@ -275,4 +275,5 @@
 #if (!(defined(TRIO_COMPILER_SUPPORTS_C99) \
  || defined(TRIO_COMPILER_SUPPORTS_UNIX01))) \
  && !defined(_WIN32_WCE)
+# ifndef floorl
 #  define floorl(x) floor((double)(x))
@@ -278,2 +279,4 @@
 #  define floorl(x) floor((double)(x))
+# endif
+# ifndef fmodl
 #  define fmodl(x,y) fmod((double)(x),(double)(y))
@@ -279,3 +282,5 @@
 #  define fmodl(x,y) fmod((double)(x),(double)(y))
+# endif
+# ifndef powl
 #  define powl(x,y) pow((double)(x),(double)(y))
 # endif
@@ -280,5 +285,6 @@
 #  define powl(x,y) pow((double)(x),(double)(y))
 # endif
+#endif
 
 #define TRIO_FABS(x) (((x) < 0.0) ? -(x) : (x))