/usr/bin/install: will not overwrite just-created '/usr/share/postgresql/13/extension/bignum--0.8.0.sql' with './/sql/bignum--0.8.0.sql'
make: *** [/usr/lib/postgresql/13/lib/pgxs/src/makefiles/pgxs.mk:233: install] Error 1

$ lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 20.04.2 LTS
Release:	20.04
Codename:	focal

$ psql --version
psql (PostgreSQL) 13.2 (Ubuntu 13.2-1.pgdg20.04+1)

$ make
cp sql/bignum.sql sql/bignum--0.8.0.sql
gcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Werror=vla -Wendif-labels -Wmissing-format-attribute -Wimplicit-fallthrough=3 -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -Wno-format-truncation -Wno-stringop-truncation -g -g -O2 -fstack-protector-strong -Wformat -Werror=format-security -fno-omit-frame-pointer -fPIC -I. -I./ -I/usr/include/postgresql/13/server -I/usr/include/postgresql/internal  -Wdate-time -D_FORTIFY_SOURCE=2 -D_GNU_SOURCE -I/usr/include/libxml2   -c -o src/bignum.o src/bignum.c
gcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Werror=vla -Wendif-labels -Wmissing-format-attribute -Wimplicit-fallthrough=3 -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -Wno-format-truncation -Wno-stringop-truncation -g -g -O2 -fstack-protector-strong -Wformat -Werror=format-security -fno-omit-frame-pointer -fPIC src/bignum.o -L/usr/lib/x86_64-linux-gnu -Wl,-Bsymbolic-functions -Wl,-z,relro -Wl,-z,now -L/usr/lib/llvm-9/lib  -Wl,--as-needed  -shared -o src/bignum.so
/usr/bin/clang-9 -Wno-ignored-attributes -fno-strict-aliasing -fwrapv -O2  -I. -I./ -I/usr/include/postgresql/13/server -I/usr/include/postgresql/internal  -Wdate-time -D_FORTIFY_SOURCE=2 -D_GNU_SOURCE -I/usr/include/libxml2  -flto=thin -emit-llvm -c -o src/bignum.bc src/bignum.c

$ sudo make install
/bin/mkdir -p '/usr/share/postgresql/13/extension'
/bin/mkdir -p '/usr/share/postgresql/13/extension'
/bin/mkdir -p '/usr/lib/postgresql/13/lib'
/bin/mkdir -p '/usr/share/doc/postgresql-doc-13/extension'
/usr/bin/install -c -m 644 .//bignum.control '/usr/share/postgresql/13/extension/'
/usr/bin/install -c -m 644 .//sql/bignum--0.8.0.sql .//sql/bignum--0.8.0.sql  '/usr/share/postgresql/13/extension/'
/usr/bin/install: will not overwrite just-created '/usr/share/postgresql/13/extension/bignum--0.8.0.sql' with './/sql/bignum--0.8.0.sql'
make: *** [/usr/lib/postgresql/13/lib/pgxs/src/makefiles/pgxs.mk:233: install] Error 1