I was looking for a UTF-8 lightweight library with a license compatible with GDCM.

I (Mathieu Malaterre) decided to pick utfcpp:
http://sourceforge.net/projects/utfcpp/

Version 2.3.4 is included here, with no changes.

Source:
http://sourceforge.net/projects/utfcpp/files/utf8cpp_2x/Release%202.3.4/utf8_v2_3_4.zip/download

SHA 256 checksum:
3373cebb25d88c662a2b960c4d585daf9ae7b396031ecd786e7bb31b15d010ef