Fix weak alias support detection

Since GCC4, it is required that when declaring a function alias,
the target function is defined in the same compilation unit.

See #2002
Signed-off-by: Ken Raffenetti <raffenet@mcs.anl.gov>

confdb/aclocal_cc.m4

@@ -368,7 +368,7 @@ pac_cv_attr_weak_alias,[
 PAC_PUSH_FLAG([CFLAGS])
 # force an error exit if the weak attribute isn't understood
 CFLAGS=-Werror
-AC_TRY_COMPILE([int foo(int) __attribute__((weak,alias("__foo")));],[int a;],
+AC_TRY_COMPILE([int __foo(int a){return 0;} int foo(int) __attribute__((weak,alias("__foo")));],[int a;],
 pac_cv_attr_weak_alias=yes,pac_cv_attr_weak_alias=no)
 # Restore original CFLAGS
 PAC_POP_FLAG([CFLAGS])])