patch 8.0.0199: compiler warnings for libcall

Problem:    Warning for an unused parameter when the libcall feature is
            disabled.  Warning for a function type cast when compiling with
            -pedantic.
Solution:   Add UNUSED.  Use a different type cast. (Damien Molinier)
diff --git a/src/os_unix.c b/src/os_unix.c
index aa3c3e5..48359d3 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -6918,7 +6918,7 @@
 	    if (argstring != NULL)
 	    {
 # if defined(USE_DLOPEN)
-		ProcAdd = (STRPROCSTR)dlsym(hinstLib, (const char *)funcname);
+		*(void **)(&ProcAdd) = dlsym(hinstLib, (const char *)funcname);
 		dlerr = (char *)dlerror();
 # else
 		if (shl_findsym(&hinstLib, (const char *)funcname,
@@ -6940,7 +6940,7 @@
 	    else
 	    {
 # if defined(USE_DLOPEN)
-		ProcAddI = (INTPROCSTR)dlsym(hinstLib, (const char *)funcname);
+		*(void **)(&ProcAddI) = dlsym(hinstLib, (const char *)funcname);
 		dlerr = (char *)dlerror();
 # else
 		if (shl_findsym(&hinstLib, (const char *)funcname,