Bug 18421 – rdmd test-case for pragma(lib) support is incompatible with GDC

`rdmd_test` includes a test-case to ensure that `rdmd` works for programs that use `pragma(lib)` to load libraries (see https://issues.dlang.org/show_bug.cgi?id=16978 for the motivation). The correctness sought here is not in any direct handling of `pragma(lib)`, but in ensuring that the way `rdmd` invokes the D compiler will allow the `pragma(lib)` call to succeed. Since GDC does not support `pragma(lib)`, the test case will fail when `gdmd` is used as the D compiler invoked by `rdmd`. As this is a fault in the compiler rather than `rdmd`, the test case can simply be disabled when testing with `gdmd` (since from `rdmd`'s point of view, it suffices to demonstrate that it works with D compilers that support `pragma(lib)`). However, this is merely a workaround, not a fix. Long-term, the test-case should be re-enabled as soon as GDC support for `pragma(lib)` is available. This issue has been created in order to ensure that this task does not get forgotten ;-)

Moved to https://github.com/dlang/tools/issues/391