The following patch fixes this (for me):
diff --git a/lang/qt/tests/Makefile.am b/lang/qt/tests/Makefile.am index 32ad6466..aedd3264 100644 --- a/lang/qt/tests/Makefile.am +++ b/lang/qt/tests/Makefile.am @@ -51,10 +51,10 @@ LDADD = ../../cpp/src/libgpgmepp.la ../src/libqgpgme.la \ ../../../src/libgpgme.la @GPGME_QT5_LIBS@ @GPG_ERROR_LIBS@ \ @GPGME_QT5TEST_LIBS@ @LDADD_FOR_TESTS_KLUDGE@ -lstdc++