Add PKG_CONFIG_SYSROOT_MAP support
Similar to PKG_CONFIG_SYSROOT_DIR, but instead of prepending the same sysroot path to all flags, prepend the sysroot corresponding to the location where the pc file was found.
e.g. PKG_CONFIG_SYSROOT_MAP=/sdk/usr/lib/pkgconfig:/sdk
will prepend
/sdk only to flags from pc files found in /sdk/usr/lib/pkgconfig and not
those found in /usr.