Allow dlopen self test to work with gcc's -fvisibility=hidden.

Allow dlopen self test to work with gcc's -fvisibility=hidden.

* libltdl/m4/libtool.m4 (_LT_TRY_DLOPEN_SELF): Declare default
visibility for the symbol we are going to test dlopen (NULL)
when a GCC version is used that understands the visibility
attribute, under the assumption that if -fvisibility=hidden
will be used, the user code will be sufficiently annotated
for visibility of needed symbols from the main executable.
* THANKS: Update.
Report by Josh Hursey against OpenMPI.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>