Fix the test for fsetxattr and strerror_r tests in CMake to work without compiling

CMake/CurlTests.c

@@ -507,30 +507,30 @@ main ()
 #ifdef HAVE_GLIBC_STRERROR_R
 #include <string.h>
 #include <errno.h>
+
+void check(char c) {}
+
 int
 main () {
-  char buffer[1024]; /* big enough to play with */
-  char *string =
-    strerror_r(EACCES, buffer, sizeof(buffer));
-    /* this should've returned a string */
-    if(!string || !string[0])
-      return 99;
-    return 0;
+  char buffer[1024];
+  // This will not compile if strerror_r does not return a char*
+  check(strerror_r(EACCES, buffer, sizeof(buffer))[0]);
+  return 0;
 }
 #endif
 #ifdef HAVE_POSIX_STRERROR_R
 #include <string.h>
 #include <errno.h>
+
+// float, because a pointer can't be implicitly cast to float
+void check(float f) {}
+
 int
 main () {
-  char buffer[1024]; /* big enough to play with */
-  int error =
-    strerror_r(EACCES, buffer, sizeof(buffer));
-    /* This should've returned zero, and written an error string in the
-       buffer.*/
-    if(!buffer[0] || error)
-      return 99;
-    return 0;
+  char buffer[1024];
+  // This will not compile if strerror_r does not return an int
+  check(strerror_r(EACCES, buffer, sizeof(buffer)));
+  return 0;
 }
 #endif
 #ifdef HAVE_FSETXATTR_6