FTBFS: gcc9 stringop-truncation and others

Same error with latest upstream, which is 4.7 at this time:
libtool: compile: x86_64-linux-gnu-gcc -DHAVE_CONFIG_H -DDEFAULT_CONFIG_FILE=\"/etc/squid/squid.conf\" -DDEFAULT_SQUID_DATA_DIR=\"/usr/share/squid\" -DDEFAULT_SQUID_CONFIG_DIR=\"/etc/squid\" -I../.. -I../../include -I../../lib -I../../src -I../../include -isystem /usr/include/mit-krb5 -I../../lib -Wdate-time -D_FORTIFY_SOURCE=2 -I/usr/include/libxml2 -Wall -Wpointer-arith -Wwrite-strings -Wmissing-prototypes -Wmissing-declarations -Wcomments -Wshadow -Werror -pipe -D_REENTRANT -g -O2 -fdebug-prefix-map=/home/ubuntu/git/packages/squid/squid=. -fstack-protector-strong -Wformat -Werror=format-security -Wall -c smbencrypt.c -fPIC -DPIC -o .libs/smbencrypt.o
In file included from /usr/include/string.h:494,
                 from ../../compat/xstring.h:13,
                 from ../../compat/compat_shared.h:225,
                 from ../../compat/compat.h:87,
                 from ../../include/squid.h:43,
                 from smblib-util.c:33:
In function ‘strncpy’,
    inlined from ‘SMB_Negotiate’ at smblib-util.c:404:9:
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:106:10: error: ‘__builtin_strncpy’ output may be truncated copying 79 bytes from a string of length 79 [-Werror=stringop-truncation]
  106 | return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In function ‘strncpy’,
    inlined from ‘SMB_Negotiate’ at smblib-util.c:427:9:
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:106:10: error: ‘__builtin_strncpy’ output may be truncated copying 79 bytes from a string of length 79 [-Werror=stringop-truncation]
  106 | return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In function ‘strncpy’,
    inlined from ‘SMB_TreeConnect’ at smblib-util.c:541:5:
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:106:10: error: ‘__builtin_strncpy’ specified bound 129 equals destination size [-Werror=stringop-truncation]
  106 | return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In function ‘strncpy’,
    inlined from ‘SMB_TreeConnect’ at smblib-util.c:542:5:
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:106:10: error: ‘__builtin_strncpy’ specified bound 20 equals destination size [-Werror=stringop-truncation]
  106 | return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
libtool: compile: x86_64-linux-gnu-gcc -DHAVE_CONFIG_H -DDEFAULT_CONFIG_FILE=\"/etc/squid/squid.conf\" -DDEFAULT_SQUID_DATA_DIR=\"/usr/share/squid\" -DDEFAULT_SQUID_CONFIG_DIR=\"/etc/squid\" -I../.. -I../../include -I../../lib -I../../src -I../../include -isystem /usr/include/mit-krb5 -I../../lib -Wdate-time -D_FORTIFY_SOURCE=2 -I/usr/include/libxml2 -Wall -Wpointer-arith -Wwrite-strings -Wmissing-prototypes -Wmissing-declarations -Wcomments -Wshadow -Werror -pipe -D_REENTRANT -g -O2 -fdebug-prefix-map=/home/ubuntu/git/packages/squid/squid=. -fstack-protector-strong -Wformat -Werror=format-security -Wall -c smbencrypt.c -o smbencrypt.o >/dev/null 2>&1
cc1: all warnings being treated as errors
make[3]: *** [Makefile:816: smblib-util.lo] Error 1
make[3]: *** Waiting for unfinished jobs....
make[3]: Leaving directory '/home/ubuntu/git/packages/squid/squid/lib/smblib'
make[2]: *** [Makefile:1047: all-recursive] Error 1
make[2]: Leaving directory '/home/ubuntu/git/packages/squid/squid/lib'
make[1]: *** [Makefile:584: all-recursive] Error 1
make[1]: Leaving directory '/home/ubuntu/git/packages/squid/squid'
make: *** [/usr/share/cdbs/1/class/makefile.mk:77: debian/stamp-makefile-build] Error 2
dpkg-buildpackage: error: debian/rules build subprocess returned exit status 2