Hi Matthias,
Running ./sage -sh -c 'pkg-config --cflags singular' with home-brew's singular installed gives:
-DSING_NDEBUG -DOM_NDEBUG -DSING_NDEBUG -DOM_NDEBUG -DSING_NDEBUG -DOM_NDEBUG -DSING_NDEBUG -DOM_NDEBUG -I/Users/zscherr/sage/develop/local/include/singular
my env is a mess, but I do source .homebrew-build-env, here is what I think are the relevant variables:
PATH=/usr/local/opt/gettext/bin:/Users/zscherr/perl5/bin:/usr/local/sbin:/usr/local/Cellar/modules/4.5.3/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/Library/TeX/texbin:/opt/X11/bin:/Library/Apple/usr/bin
PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig:/usr/local/opt/sqlite/lib/pkgconfig:/usr/local/opt/readline/lib/pkgconfig:/usr/local/opt/openssl/lib/pkgconfig:/usr/local/opt/openblas/lib/pkgconfig:/usr/local/lib/pkgconfig:
LIBRARY_PATH=/usr/local/opt/gcc@9/lib/gcc/9:/usr/local/opt/readline/lib:/usr/local/lib
CPATH=/usr/local/opt/readline/include:/usr/local/include
ACLOCAL_PATH=/usr/local/opt/gettext/share/aclocal:
As expected, if I brew uninstall singular then running ./sage -sh -c 'pkg-config --cflags singular' gives
-DSING_NDEBUG -DOM_NDEBUG -DSING_NDEBUG -DOM_NDEBUG -DSING_NDEBUG -DOM_NDEBUG -DSING_NDEBUG -DOM_NDEBUG -I/Users/zscherr/sage/develop/local/include/singular
I was able to build beta 11 so I can try to git bisect if you think that would be useful.