Bug#919573: sagemath ftbfs with ld --as-needed as the default

[Matthias Klose]

Package: src:sagemath
Version: 8.6-2
Severity: important
Tags: sid buster

sagemath ftbfs with ld --as-needed as the default

the build log shows in various locations:

[...]
[sagelib-8.6] g++ -pthread -shared -Wl,-O1 -Wl,-Bsymbolic-functions
-Wl,-Bsymbolic-functions -Wl,-z,relro -fno-strict-aliasing -DNDEBUG -g -fwrapv
-O2 -Wall -Wstrict-prototypes -Wdate-time -D_FORTIFY_SOURCE=2 -g
-fdebug-prefix-map=/build/python2.7-bqwtKO/python2.7-2.7.15=.
-fstack-protector-strong -Wformat -Werror=format-security
-L/<<PKGBUILDDIR>>/debian/build/usr/lib -Wl,-rpath,/usr/lib/x86_64-linux-gnu/gap
-g -O2 -fdebug-prefix-map=/<<PKGBUILDDIR>>=. -fstack-protector-strong -Wformat
-Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2
build/temp.linux-x86_64-2.7/build/cythonized/sage/matrix/matrix_modn_sparse.o
-L/<<PKGBUILDDIR>>/debian/build/usr/lib -L/usr/lib/x86_64-linux-gnu/gap
-llinbox-1.5.2 -llinboxsage-1.5.2 -lntl -liml -lblas -llapack -lgivaro -lflint
-lmpfr -lgmp -lgmpxx -lstdc++ -o
build/lib.linux-x86_64-2.7/sage/matrix/matrix_modn_sparse.so -lpari
[...]

the object file appears *after* the libraries on the command line, so nothing is
linked.  I didn't grok sage/setup.py where exactly that occurs, but you always
should place the libraries after the object files on the command line.

You can test that by setting

export DEB_LDFLAGS_MAINT_APPEND = -Wl,--as-needed

Please forward that upstream.