Otherwise, CppunitTest_sw_odfexport (which happens to load both libwps and
libnumbertext) on Linux --enable-dbgutil would cause a crash when
testSpellOutNumberingTypes calls into libnumbertext's use of std::regexp, which
instantiates a std::__detail::_NFA<...> with std::__debug::vector<...> members,
but happens to bind to libwps' (exported, inline) functions to destroy that
_NFA again, and libwps' view of std::__detail::_NFA<...> uses non-debug plain
std::vector<...> members.

(Copy/pastes the code to enable _GBLICXX_DEBUG as already used in various other
external/ExternalProject_*.mk, incl. ExternalProject_libnumbertext.mk.  This
mess should be cleaned up.)

(It looks like a libstdc++ bug to me that such instances of _NFA with different
members due to debug mode nevertheless have the same mangled names.)

Change-Id: I92a41a39c5fe6ae5eb71855e32d929690219f1d1