Replying my own question:
The following tweaks worked for compiling the libs:
Change mysql.pro to:
TARGET = qsqlmysql
HEADERS += $$PWD/qsql_mysql_p.h
SOURCES += $$PWD/qsql_mysql.cpp $$PWD/main.cpp
LIBS += -L/usr/local/mysql/include
#QMAKE_USE += mysql
OTHER_FILES += mysql.json
PLUGIN_CLASS_NAME = QMYSQLDriverPlugin
include(../qsqldriverbase.pri)
Then run qmake like this:
/Users/daniel/Qt/5.9.1/clang_64/bin/qmake "INCLUDEPATH+=/usr/local/mysql" "LIBS+=-L/usr/local/mysql/lib -lmysqlclient" "INCLUDEPATH+=/usr/local/mysql/include" mysql.pro
make and install:
make
make install
My Application is still not using the Mysql-driver, but that the next step, I presume.
Edit: make install line added