FindPackageModeMakefileTest: Explicitly use C linkage

This explicitly uses C linkage for the library code used by the test to work
around some compilers (Solaris Studio 12.4) that implicitly assume incorrect
linkage information.  Since something else entirely is being tested here,
this allows the test to proceed as needed on the affected platforms.

Tests/FindPackageModeMakefileTest/foo.cpp

@@ -1,3 +1,5 @@
+#include "foo.h"
+
 int foo()
 {
   return 1477;

Tests/FindPackageModeMakefileTest/foo.h

@@ -1,6 +1,14 @@
 #ifndef FOO_H
 #define FOO_H
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 int foo();
 
+#ifdef __cplusplus
+} // extern "C"
+#endif
+
 #endif