Allow proper compiling of libjpeg for windows.

This expands on #777 to work with jconfig.h properly.

To facilitate these operations, a new macro was added for doing compile tests.

Note that this commit does NOT bring in the other files associated with this version of libjpeg.

At some point, it would be desirable to automatically get the files as part of the cmake configuration step, similar to ffmpeg.

CMake/TestCode/test_broken.c

@@ -0,0 +1,3 @@
+/* Borrowed from libjpeg. */
+typedef struct undefined_structure * undef_struct_ptr;
+

CMake/TestCode/test_external.c

@@ -0,0 +1,11 @@
+#include <stdio.h>
+
+int very_long_name_here_one() { return 0; }
+int very_long_name_here_two() { return 1; }
+
+int main() {
+  printf("%d\n", very_long_name_here_one());
+  printf("%d\n", very_long_name_here_two());
+  return 0;
+}
+

CMake/TestCode/test_prototype.c

@@ -0,0 +1,17 @@
+/* Borrowed from Wikipedia: Function prototype page. */
+#include <stdio.h>
+
+int myfunction(int n);
+
+int main(void) {
+  printf("%d\n", myfunction()); /* Intentional: we want this to fail. */
+  return 0;
+}
+
+int myfunction(int n) {
+  if (n == 0) {
+    return 1;
+  }
+  return n * myfunction(n - 1);
+}
+