[gentoo-user] matplotlib build fails: link using /usr/lib instead of /usr/lib64

[Grant Edwards]

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]