cannot create extension

I cannot create the bignum extension. I need it for the pg-cert extension for postgresql-9.6

git clone https://github.com/beargiles/pg-bignum
cd pg-bignum

cp sql/bignum.sql sql/bignum--0.8.0.sql
gcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -g -g -O2 -fdebug-prefix-map=/build/postgresql-9.6-YRPps9/postgresql-9.6-9.6.6=. -specs=/usr/share/dpkg/no-pie-compile.specs -fstack-protector-strong -Wformat -Werror=format-security -I/usr/include/mit-krb5 -fPIC -pie -fno-omit-frame-pointer -fPIC -I. -I./ -I/usr/include/postgresql/9.6/server -I/usr/include/postgresql/internal -Wdate-time -D_FORTIFY_SOURCE=2 -D_GNU_SOURCE -I/usr/include/libxml2 -I/usr/include/tcl8.6 -c -o src/bignum.o src/bignum.c
gcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -g -g -O2 -fdebug-prefix-map=/build/postgresql-9.6-YRPps9/postgresql-9.6-9.6.6=. -specs=/usr/share/dpkg/no-pie-compile.specs -fstack-protector-strong -Wformat -Werror=format-security -I/usr/include/mit-krb5 -fPIC -pie -fno-omit-frame-pointer -fPIC -L/usr/lib/x86_64-linux-gnu -specs=/usr/share/dpkg/no-pie-link.specs -Wl,-z,relro -Wl,-z,now -Wl,--as-needed -L/usr/lib/mit-krb5 -L/usr/lib/x86_64-linux-gnu/mit-krb5 -Wl,--as-needed -shared -o src/bignum.so src/bignum.o
jotpe@hobbit:~/postgresql/contrib/pg-bignum$ sudo make install
/bin/mkdir -p '/usr/share/postgresql/9.6/extension'
/bin/mkdir -p '/usr/share/postgresql/9.6/extension'
/bin/mkdir -p '/usr/lib/postgresql/9.6/lib'
/bin/mkdir -p '/usr/share/doc/postgresql-doc-9.6/extension'
/usr/bin/install -c -m 644 .//bignum.control '/usr/share/postgresql/9.6/extension/'
/usr/bin/install -c -m 644 .//sql/bignum--0.8.0.sql .//sql/bignum--0.8.0.sql '/usr/share/postgresql/9.6/extension/'
/usr/bin/install: Neu erstelltes '/usr/share/postgresql/9.6/extension/bignum--0.8.0.sql' wird nicht mit './/sql/bignum--0.8.0.sql' überschrieben.
/usr/lib/postgresql/9.6/lib/pgxs/src/makefiles/pgxs.mk:114: die Regel für Ziel „install“ scheiterte
make: *** [install] Fehler 1

It sais there was an error "/usr/bin/install: Newly created' /usr/share/postgresql/9.6/extension/bignum--0.8.0. sql' will not be overwritten with'. //sql/bignum--0.8.0. sql'."

And trying to call, leads to an error.
create extension bignum;
FEHLER: konnte nicht auf Datei »bignum« zugreifen: Datei oder Verzeichnis nicht gefunden
"ERROR: could not access file "bignum": File or directory not found"