Headers and Linking
The documentation under Headers and Linking which can be found online states:
PKG_CHECK_MODULES([MYAPP], [gtkmm-3.0 >= 3.8.0])
This checks for the presence of gtkmm and defines MYAPP_LIBS and MYAPP_CFLAGS for use in your Makefile.am files.
While technically correct in that you could name the gtkmm prefix "MYAPP", it would probably be more appropriate to encourage people to use [GTK] as the prefix for clarity in Makefile.am.