Fix bindir and dlopen tests for C++ compilers (CC=g++).

[Ralf Wildenhues]

I've built Libtool and run the testsuites with CC=g++ on GNU/Linux,
the following patch fixes the fallout.  Pushed as obvious.

Thanks,
Ralf

    Fix bindir and dlopen tests for C++ compilers (CC=g++).
    
    * tests/bindir.at (bindir basic lib test, bindir install tests):
    Include stdlib.h rather than declaring abort.
    * tests/lt_dlopenext.at (lt_dlopenext error messages): Use
    `extern "C"' annotation for dlsym'ed function in C++ mode.

diff --git a/tests/bindir.at b/tests/bindir.at
index 859b1c4..e49853d 100644
--- a/tests/bindir.at
+++ b/tests/bindir.at
@@ -128,9 +128,9 @@ int bar (void) { return y + baz ();}
 ]])
 
 AT_DATA([main.c],[[
+#include <stdlib.h>
 extern int baz (void);
 extern int bar (void);
-extern void abort (void);
 int main() {
 if (baz () + bar () - 25) abort ();
 return 0;
@@ -237,9 +237,9 @@ int bar (void) { return y + baz ();}
 ]])
 
 AT_DATA([main.c],[[
+#include <stdlib.h>
 extern int baz (void);
 extern int bar (void);
-extern void abort (void);
 int main() {
 if (baz () + bar () - 25) abort ();
 return 0;
diff --git a/tests/lt_dlopenext.at b/tests/lt_dlopenext.at
index e83f723..84dc8e5 100644
--- a/tests/lt_dlopenext.at
+++ b/tests/lt_dlopenext.at
@@ -41,6 +41,9 @@ AT_DATA([simple-module.c],
 [[
 #include <stdio.h>
 
+#ifdef __cplusplus
+extern "C"
+#endif
 int
 run (const char *argument)
 {