I tried to emerge matplotlib today, and it failed because it's linking in 32-bit libraries instead of 64 bit ones:
x86_64-pc-linux-gnu-g++ -shared -Wl,-O1 -Wl,--as-needed -march=native -O2 -pipe -fno-strict-aliasing -DNDEBUG /var/tmp/portage/dev-python/matplotlib-2.2.2-r1/work/matplotlib-2.2.2-python3_7/build/temp.linux-x86_64-3.7/src/mplutils.o /var/tmp/portage/dev-python/matplotlib-2.2.2-r1/work/matplotlib-2.2.2-python3_7/build/temp.linux-x86_64-3.7/src/py_converters.o /var/tmp/portage/dev-python/matplotlib-2.2.2-r1/work/matplotlib-2.2.2-python3_7/build/temp.linux-x86_64-3.7/src/_backend_agg.o /var/tmp/portage/dev-python/matplotlib-2.2.2-r1/work/matplotlib-2.2.2-python3_7/build/temp.linux-x86_64-3.7/src/_backend_agg_wrapper.o /var/tmp/portage/dev-python/matplotlib-2.2.2-r1/work/matplotlib-2.2.2-python3_7/build/temp.linux-x86_64-3.7/extern/agg24-svn/src/agg_bezier_arc.o /var/tmp/portage/dev-python/matplotlib-2.2.2-r1/work/matplotlib-2.2.2-python3_7/build/temp.linux-x86_64-3.7/extern/agg24-svn/src/agg_curves.o /var/tmp/portage/dev-python/matplotlib-2.2.2-r1/work/matplotlib-2.2.2-python3_7/bui ld/temp.linux-x86_64-3.7/extern/agg24-svn/src/agg_image_filters.o /var/tmp/portage/dev-python/matplotlib-2.2.2-r1/work/matplotlib-2.2.2-python3_7/build/temp.linux-x86_64-3.7/extern/agg24-svn/src/agg_trans_affine.o /var/tmp/portage/dev-python/matplotlib-2.2.2-r1/work/matplotlib-2.2.2-python3_7/build/temp.linux-x86_64-3.7/extern/agg24-svn/src/agg_vcgen_contour.o /var/tmp/portage/dev-python/matplotlib-2.2.2-r1/work/matplotlib-2.2.2-python3_7/build/temp.linux-x86_64-3.7/extern/agg24-svn/src/agg_vcgen_dash.o /var/tmp/portage/dev-python/matplotlib-2.2.2-r1/work/matplotlib-2.2.2-python3_7/build/temp.linux-x86_64-3.7/extern/agg24-svn/src/agg_vcgen_stroke.o /var/tmp/portage/dev-python/matplotlib-2.2.2-r1/work/matplotlib-2.2.2-python3_7/build/temp.linux-x86_64-3.7/extern/agg24-svn/src/agg_vpgen_segmentator.o -L/usr/lib -L/usr/lib64 -L/usr/lib64 -lfreetype -lpython3.7m -o /var/tmp/portage/dev-python/matplotlib-2.2.2-r1/work/matplotlib-2.2.2-python3_7/build/lib/matplotlib/backends/_backend_agg. cpython-37m-x86_64-linux-gnu.so /usr/lib/gcc/x86_64-pc-linux-gnu/9.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: skipping incompatible /usr/lib/libm.so when searching for -lm /usr/lib/gcc/x86_64-pc-linux-gnu/9.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: skipping incompatible /usr/lib/libm.a when searching for -lm /usr/lib/gcc/x86_64-pc-linux-gnu/9.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: skipping incompatible /usr/lib/libc.so when searching for -lc /usr/lib/gcc/x86_64-pc-linux-gnu/9.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: skipping incompatible /usr/lib/libc.a when searching for -lc error: command 'x86_64-pc-linux-gnu-g++' failed with exit status 1 What would cause that? It doesn't appear that it's being build for both 32-bit and 64-bit targets: These are the packages that would be merged, in order: Calculating dependencies... done! [ebuild N ] dev-python/matplotlib-2.2.2-r1::gentoo USE="-cairo (-doc) -examples -excel -gtk2 -gtk3 -latex -qt5 -test -tk -wxwidgets" PYTHON_TARGETS="python2_7 python3_7 -python3_6" 0 KiB Total: 1 package (1 new), Size of downloads: 0 KiB Would you like to merge these packages? [Yes/No]