Make function checks more robust within shared libs

Previous attempt to error at link like was with

1747cdf

this however causes issues with newer clang where it detects
the assignment as -Wint-conversion warning which is treated at error
and builds with clang fail. So this is an attempt to instruct
linker explicitly to error out if the symbol is not found during link
time when building a shared library, this fixes both the problems
as reported in

#23

as well as

#33 (comment)

Signed-off-by: Khem Raj <[email protected]>

Makefile.PL

@@ -163,7 +163,8 @@ main ()
 #if defined (__stub_$f) || defined (__stub___$f)
 choke me
 #else
-f = $f ();
+f = $f;
+f();
 #endif
 
   ;
@@ -173,7 +174,7 @@ ESQ
 
   close(TST);
   print "Looking for $f()" . "." x (13-length($f)) . " ";
-  if (system("$cfg{'cc'} $flags $funcs{$f} functest_$f.c > functest_$f.log 2>&1")) {
+  if (system("$cfg{'cc'} $flags -Wl,--no-undefined $funcs{$f} functest_$f.c > functest_$f.log 2>&1")) {
     print "not found.\n";
   } else {
     $define{"-DHAVE_\U$f"} = undef;