checking if expat.h is valid C++ header... yes
checking for XML_ParserCreate in -lexpat... yes
checking for mspack.h... no
checking for GTK+ version...
checking for pkg-config... /usr/bin/pkg-config
checking for GTK+ - version >= 2.0.0... no

• • • Could not run GTK+ test program, checking why...
    • The test program failed to compile or link. See the file config.log for the
    • exact error that occured. This usually means GTK+ is incorrectly installed.
      configure: error:
      The development files for GTK+ were not found. For GTK+ 2, please
      ensure that pkg-config is in the path and that gtk+-2.0.pc is
      installed. For GTK+ 1.2 please check that gtk-config is in the path,
      and that the version is 1.2.3 or above. Also check that the
      libraries returned by 'pkg-config gtk+-2.0 --libs' or 'gtk-config
      --libs' are in the LD_LIBRARY_PATH or equivalent.

!!! Please attach the following file when seeking support:
!!! /var/tmp/ptmp/portage/x11-libs/wxGTK-2.8.12.1-r1/work/wxPython-src-2.8.12.1/wxgtk_build/config.log

• ERROR: x11-libs/wxGTK-2.8.12.1-r1 failed (configure phase):
• econf failed
• Call stack:
• ebuild.sh, line 93: Called src_configure
• environment, line 3039: Called econf '-enable-compat26' 'enable-shared' 'enable-unicode' 'with-regex=builtin' 'with-zlib=sys' 'with-expat=sys' 'disable-debug' 'disable-precomp-headers' 'without-odbc' 'with-sdl' 'with-libtiff=sys' 'enable-graphics_ctx' 'enable-gui' 'with-libpng=sys' 'with-libxpm=sys' 'with-libjpeg=sys' 'enable-mediactrl' 'enable-opengl' 'with-opengl' 'without-gnomeprint' '-without-gnomevfs'
• phase-helpers.sh, line 521: Called die
• The specific snippet of code:
• die "econf failed"

1. genlop x11-libs/gtk+-2.24.22

• x11-libs/gtk+

Sun Feb 23 11:18:14 2014 >>> x11-libs/gtk+-2.24.22

1. genlop dev-util/pkgconf

• dev-util/pkgconf

Sun Feb 23 07:21:20 2014 >>> dev-util/pkgconf-0.9.3

1. pkg-config gtk+-2.0 --libs

1. pkg-config --libs gtk+-2.0
   -lgtk-x11-2.0 -lgdk-x11-2.0 -lpangocairo-1.0 -latk-1.0 -lcairo -lgdk_pixbuf-2.0 -lgio-2.0 -lpangoft2-1.0 -lpango-1.0 -lgobject-2.0 -lglib-2.0 -lfontconfig -lfreetype

After remerge of dev-util/pkgconf with USE="-strict" wxGTK successfully finds gtk+-2.0 files and merges cleanly.

When I first read the description for USE flag strict I thought it was desirable (perhaps even necessary) for drop-in replacement capability. It didn't occur to me that one of the packages to fail would be wxGTK which seems to be almost unavoidable on a desktop setup.