So u_strToLower is a function that is part of Unicode support. A grep on the QtCore source shows:
corelib/text/qlocale_icu.cpp:// caseFunc can either be u_strToUpper or u_strToLower
corelib/text/qlocale_icu.cpp: bool err = qt_u_strToCase(str, &out, localeID, u_strToLower);
u_strToLower is part of the unicode libraries. I’d also check libicu, libicui18n, libicuuc – although ldd says you have them.