[cairo] Making configure error message more clear

[Behdad Esfahbod]

On 03/25/2010 05:53 PM, Bill Spitzak wrote:
> 
>>   - If we only use $PREFIX/lib/pkgconfig if PKG_CONFIG_PATH is not
>> set, then
>> user setting PKG_CONFIG_PATH to another location will break the build
>> since
>> they now have to add $PREFIX/lib/pkgconfig manually.
> 
> I don't understand what is wrong with this solution. If they are setting
> it, then they have probably set it correctly, thus it either includes
> this directory, or that directory is not needed.

For example, my .bash_profile always adds ~/.local/lib/pkg-config to
PKG_CONFIG_PATH.  Doesn't mean I have set /usr/local/lib/pkg-config in it.

behdad