prefer PKG_CONFIG_PATH over default.

Just like pkg-config:

 iscah% setenv PKG_CONFIG_PATH /tmp
 iscah% pkg-config | & head
 PKG_CONFIG_DEBUG_SPEW variable enabling debug spew
 Adding directory '/tmp' from PKG_CONFIG_PATH
 Adding directory '/usr/local/lib/x86_64-linux-gnu/pkgconfig' from PKG_CONFIG_PATH
 Adding directory '/usr/local/lib/pkgconfig' from PKG_CONFIG_PATH
 Adding directory '/usr/local/share/pkgconfig' from PKG_CONFIG_PATH
 Adding directory '/usr/lib/x86_64-linux-gnu/pkgconfig' from PKG_CONFIG_PATH
 Adding directory '/usr/lib/pkgconfig' from PKG_CONFIG_PATH
 Adding directory '/usr/share/pkgconfig' from PKG_CONFIG_PATH
 Global variable definition 'pc_sysrootdir' = '/'
 Global variable definition 'pc_top_builddir' = '$(top_builddir)'