lisp/ob-C.el: replace %f with %s to prevent unneeded rounding

* lisp/ob-C.el (org-babel-C-val-to-C-type): Floats should be printed
as string literals to prevent rounding introduced by %f format.

* testing/lisp/test-ob-C.el (ob-C/float-var): Test that floats are not
rounded when passed as an org :var.

lisp/ob-C.el

@@ -339,7 +339,7 @@ FORMAT can be either a format string or a function which is called with VAL."
 	 (type
 	  (pcase basetype
 	    (`integerp '("int" "%d"))
-	    (`floatp '("double" "%f"))
+	    (`floatp '("double" "%s")) ;; %f rounds, use %s to print the float literally
 	    (`stringp
 	     (list
 	      (if (eq org-babel-c-variant 'd) "string" "const char*")

testing/lisp/test-ob-C.el

@@ -95,6 +95,17 @@
 		      (org-babel-next-src-block 10)
 		      (should (= 42 (org-babel-execute-src-block))))))
 
+(ert-deftest ob-C/float-var ()
+  "Test that floats are passed without unnecessary rounding."
+  (if (executable-find org-babel-C++-compiler)
+      (org-test-with-temp-text 
+"#+source: float_var
+#+begin_src cpp :var x=1.123456789012345678 :includes \"<iostream>\" :results silent
+double y = 1.123456789012345678;
+std::cout << (x == y);
+#+end_src"
+(should (= 1 (org-babel-execute-src-block))))))
+
 (ert-deftest ob-C/table ()
   "Test of a table output"
   (if (executable-find org-babel-C++-compiler)