Utility: make compiled resource files usable with compatibility branch.

For e.g. x86 NaCl you will probably use native `corrade-rc` from Corrade
master to compile resource files. The NaCl toolchain is GCC 4.4 which
doesn't know about nullptr yet, thus `compatibility` branch with
`corradeCompatibility.h` header needs to be used.

Cherry-picked from 42a15cd718bbb162a2fee066edf4c0748bde477d and
partially also from e3e7a79229bfe71176e755e04378f8972384b43b (both in
compatibility branch).
Link `dl` on Unix, but not on NaCl newlib.

Partially reverts (undocumented) change done in
d1f87db6577003ed2b893e2891ab169e2ea3abbc. NaCl is understood as unix, so
simple `UNIX OR CORRADE_TARGET_NACL_GLIBC` won't work, as it will match
`CORRADE_TARGET_NACL_NEWLIB` also.