Freetype detection fails on Solaris sparcv9 when using devkit

When using a devkit on Solaris sparcv9, configure won't find freetype even though it's in the sysroot of the devkit. This is because the logic that looks for freetype inside the isa dir "/sparcv9" is only working for amd64. This needs to be generalized and use the ISADIR variable instead of only looking in /amd64.