favor /usr/local over /usr by default

compatability with the real pkg-config:

 iscah% ./pkg-config | & head
 PKG_CONFIG_DEBUG_SPEW variable enabling debug spew
 Adding directory '/usr/local/lib/pkgconfig' from PKG_CONFIG_PATH
 Adding directory '/usr/local/share/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)'
 no output option set, defaulting to --exists
 Error printing disabled by default due to use of output options --exists, --atleast/exact/max-version or no output option at all. Value of --print-errors: 1
 Error printing enabled