bpo-35081: Move dtoa.h header to the internal C API (GH-18489)

Move the dtoa.h header file to the internal C API as pycore_dtoa.h:
it only contains private functions (prefixed by "_Py").

The math and cmath modules must now be compiled with the
Py_BUILD_CORE macro defined.
diff --git a/Modules/Setup b/Modules/Setup
index 983fa01..40266a1 100644
--- a/Modules/Setup
+++ b/Modules/Setup
@@ -167,8 +167,8 @@
 # Modules that should always be present (non UNIX dependent):
 
 #array arraymodule.c	# array objects
-#cmath cmathmodule.c _math.c # -lm # complex math library functions
-#math mathmodule.c _math.c # -lm # math library functions, e.g. sin()
+#cmath cmathmodule.c _math.c -DPy_BUILD_CORE_MODULE # -lm # complex math library functions
+#math mathmodule.c _math.c -DPy_BUILD_CORE_MODULE # -lm # math library functions, e.g. sin()
 #_contextvars _contextvarsmodule.c  # Context Variables
 #_struct _struct.c	# binary structure packing/unpacking
 #_weakref _weakref.c	# basic weak reference support