PKG_CONFIG_PATH error with xfce goodies

[joe_schmoe]

Hi all

Not sure if this is an Xfce thing or a more general Slackware thing, so 
let me eliminate the possibility of it being an Xfce thing first:

I am wanting to install some Xfce panel goodies, but during the 
./configure stage of the installation an error message, this is spat out:

######################## Code #################################
checking for xfce4-panel-1.0 >= 4.1.90... Requested 'xfce4-panel-1.0 >= 
4.1.90' but version of xfce4-panel is 4.0.5

configure: error: Library requirements (xfce4-panel-1.0 >= 4.1.90) not 
met; consider adjusting the PKG_CONFIG_PATH environment variable if your 
libraries are in a nonstandard prefix so pkg-config can find them.
####################### End Code ##############################

I have tried to set this path variable BEFORE invoking ./configure 
inside the decompressed tarball directory

$export PKG_CONFIG_PATH=/usr/lib/pkgconfig  # where the requisite
                                             # library resides

But, no dice.
To the best of my knowledge, and after confirming this with a couple of 
Googles, this should have fixed the problem. Am I misinformed, or is 
this more likely to be a Slackware issue?

Cheers

/j