In gucharmap-3.3.1, there is a file named gucharmap.pc.in.
The gucharmap.pc.in has a line, 
Requires.private: @GCONFPKGS@
but GCONFPKGS is not replaced in gucharmap-2.90.pc

$ pkg-config --cflags gucharmap-2.90
Package @GCONFPKGS@ was not found in the pkg-config search path.
Perhaps you should add the directory containing `@GCONFPKGS@.pc'
to the PKG_CONFIG_PATH environment variable
Package '@GCONFPKGS@', required by 'gucharmap', not found

I think the line "Requires.private: @GCONFPKGS@" is not needed.