In reply to @Ericson2314:matrix.org
rhelmot: Do you know what the default -std flag is? the failing build didn't seem to have a -std=... but I need to provide one manually in the shell invoking $CC to reproduce the error

error: Cannot build '/nix/store/gm5win3imb4ira9xihqjgmkw6grwwz96-flex-x86_64-unknown-freebsd-2.6.4.drv'.
       Reason: builder failed with exit code 2.
       Output paths:
         /nix/store/2zahnxwrgj39cs2asrwgi0hm7bjilknw-flex-x86_64-unknown-freebsd-2.6.4
       Last 25 log lines:
       > gcc -DHAVE_CONFIG_H -I.  -DLOCALEDIR=\"/nix/store/2zahnxwrgj39cs2asrwgi0hm7bjilknw-flex-x86_64-unknown-freebsd-2.6.4/share/locale\"   -g -O2 -c -o stage1flex-sym.o `test -f 'sym.c' || echo './'`sym.c
       > gcc -DHAVE_CONFIG_H -I.  -DLOCALEDIR=\"/nix/store/2zahnxwrgj39cs2asrwgi0hm7bjilknw-flex-x86_64-unknown-freebsd-2.6.4/share/locale\"   -g -O2 -c -o stage1flex-tables.o `test -f 'tables.c' || echo './'`tables.c
       > gcc -DHAVE_CONFIG_H -I.  -DLOCALEDIR=\"/nix/store/2zahnxwrgj39cs2asrwgi0hm7bjilknw-flex-x86_64-unknown-freebsd-2.6.4/share/locale\"   -g -O2 -c -o stage1flex-tables_shared.o `test -f 'tables_shared.c' || echo './'`tables_shared.c
       > gcc -DHAVE_CONFIG_H -I.  -DLOCALEDIR=\"/nix/store/2zahnxwrgj39cs2asrwgi0hm7bjilknw-flex-x86_64-unknown-freebsd-2.6.4/share/locale\"   -g -O2 -c -o stage1flex-tblcmp.o `test -f 'tblcmp.c' || echo './'`tblcmp.c
       > gcc -DHAVE_CONFIG_H -I.  -DLOCALEDIR=\"/nix/store/2zahnxwrgj39cs2asrwgi0hm7bjilknw-flex-x86_64-unknown-freebsd-2.6.4/share/locale\"   -g -O2 -c -o stage1flex-yylex.o `test -f 'yylex.c' || echo './'`yylex.c
       > gcc -DHAVE_CONFIG_H -I.  -DLOCALEDIR=\"/nix/store/2zahnxwrgj39cs2asrwgi0hm7bjilknw-flex-x86_64-unknown-freebsd-2.6.4/share/locale\"   -g -O2 -c -o ../lib/stage1flex-malloc.o `test -f '../lib/malloc.c' || echo './'`../lib/malloc.c
       > ../lib/malloc.c:6:12: warning: conflicting types for built-in function 'malloc'; expected 'void *(long unsigned int)' [-Wbuiltin-declaration-mismatch]
       >     6 |      void *malloc ();
       >       |            ^~~~~~
       > ../lib/malloc.c:5:1: note: 'malloc' is declared in header '<stdlib.h>'
       >     4 |      #include <sys/types.h>
       >   +++ |+#include <stdlib.h>
       >     5 |
       > ../lib/malloc.c: In function 'rpl_malloc':
       > ../lib/malloc.c:16:15: error: too many arguments to function 'malloc'; expected 0, have 1
       >    16 |        return malloc (n);
       >       |               ^~~~~~  ~
       > ../lib/malloc.c:6:12: note: declared here
       >     6 |      void *malloc ();
       >       |            ^~~~~~
       > make[2]: *** [Makefile:1448: ../lib/stage1flex-malloc.o] Error 1
       > make[2]: Leaving directory '/build/flex-2.6.4/src'
       > make[1]: *** [Makefile:581: all] Error 2
       > make[1]: Leaving directory '/build/flex-2.6.4/src'
       > make: *** [Makefile:543: all-recursive] Error 1

rhelmot: last one:

Loading lib/JSON.pm failed. No B module?
perl says : Can't load module B, dynamic loading not available in this perl.
  (You may need to build a new perl executable which either supports
  dynamic loading or has the B module statically linked into it.)
 at lib/JSON/backportPP.pm line 1527.
BEGIN failed--compilation aborted at /nix/store/rb2rzqpypp3rfv35v9fiy9sja0zr92lp-perl-x86_64-unknown-freebsd-5.42.0/lib/perl5/5.42.0/x86_64-linux/B.pm line 33.
Compilation failed in require at lib/JSON/backportPP.pm line 1527.
BEGIN failed--compilation aborted at lib/JSON/backportPP.pm line 1567.
Compilation failed in require at (eval 9) line 1.
BEGIN failed--compilation aborted at (eval 9) line 1.
 at (eval 8) line 1.
Compilation failed in require at (eval 8) line 1.
Setting environmental variable 'PERL_DL_NONLAZY' to 0 may help.
No Makefile created.
Welcome to JSON (v.4.10)
=============================

 ** BACKWARD INCOMPATIBILITY **

Since version 2.90, stringification (and string comparison) for
JSON::true and JSON::false has not been overloaded. It shouldn't
matter as long as you treat them as boolean values, but a code that
expects they are stringified as "true" or "false" doesn't work as
you have expected any more.

    if (JSON::true eq 'true') {  # now fails

    print "The result is  now."; # => The result is 1 now.

And now these boolean values don't inherit JSON::Boolean, either.
When you need to test a value is a JSON boolean value or not, use
JSON::is_bool function, instead of testing the value inherits
a particular boolean class or not.

Checking if your kit is complete...
Looks good
JSON 2.5 is required for $ENV{PERL_JSON_BACKEND} = 'JSON::backportPP'
 at /nix/store/rb2rzqpypp3rfv35v9fiy9sja0zr92lp-perl-x86_64-unknown-freebsd-5.42.0/lib/perl5/5.42.0/CPAN/Meta.pm line 616.