* Avoid a space in the UDL definition except on GCC before 4.9
Clang 18 has grown a warning about the space being deprecated which
is enabled by default in their nightly binaries. However GCC before 4.9
will reject the UDL definition unless there is a space there, so we need
to keep the space conditionally for it.
* Remove UDLs on GCC before 4.9 to simplify things
GCC before 4.9 rejects the syntax that is now
rejected on more modern compilers.
* Disable compile-error-test on GCC < 4.9
This avoids the UDL tests failing as GCC < 4.9 can not parse UDLs
without a space, but the space is malformed in modern compilers.
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