can be seen in blahtexml-0.9-Makefile.patch use: PKG_CONFIG ?= pkg-config
These direct calls seems to be quite common, if I grep through the patches available in the tree. Maybe it might be worth to create a tracker bug and add all other packages with direct calls to pkg-config there, if this is a problem for you? Feel free to fix it yourself, as I might not have the time in the next days.
Yes, others do it wrong as well.
+ 26 Dec 2013; Julian Ospald <hasufell@gentoo.org> blahtexml-0.9.ebuild, + files/blahtexml-0.9-Makefile.patch: + respect PKG_CONFIG wrt #478514, add ~x86