How to install library in a specify directory?

[Lyre]

For example, I'm writing a lib named mylib. The library contain a .pc file:
instdir = ${libdir}/pkgconfig
inst_DATA = mylib.pc

And I want that *.so goes in /usr/lib/mylib/ and *.pc goes in
/usr/lib/pkgconfig/

./configure --libdir=/usr/lib/mylib will install *.so into correct
direcotry, however *.pc will goes in /usr/lib/mylib/pkgconfig.

set instdir = ${libdir}/../pkgconfig seems to works, but it looks quite
strange, Is there a better solution?