[user@devbox ~]$ uname -a
FreeBSD devbox 12.0-RELEASE FreeBSD 12.0-RELEASE r341666 GENERIC  amd64

[user@devbox ~]$ gem --version
2.7.8

[user@devbox ~]$ ruby --version
ruby 2.4.5p335 (2018-10-18 revision 65137) [x86_64-freebsd12.0]

[user@devbox ~]$ gem install bcrypt
Building native extensions. This could take a while...
ERROR:  Error installing bcrypt:
	ERROR: Failed to build gem native extension.

    current directory: /usr/home/user/.rvm/gems/ruby-2.4.5/gems/bcrypt-3.1.12/ext/mri
/usr/home/user/.rvm/rubies/ruby-2.4.5/bin/ruby -r ./siteconf20181229-84104-u51ca4.rb extconf.rb
creating Makefile

current directory: /usr/home/user/.rvm/gems/ruby-2.4.5/gems/bcrypt-3.1.12/ext/mri
make "DESTDIR=" clean

current directory: /usr/home/user/.rvm/gems/ruby-2.4.5/gems/bcrypt-3.1.12/ext/mri
make "DESTDIR="
compiling bcrypt_ext.c
In file included from bcrypt_ext.c:2:
./ow-crypt.h:19:14: error: conflicting types for 'crypt_r'
extern char *crypt_r(__CONST char *key, __CONST char *setting, void *data);
             ^
/usr/include/unistd.h:499:7: note: previous declaration is here
char    *crypt_r(const char *, const char *, struct crypt_data *);
         ^
bcrypt_ext.c:17:25: warning: implicit conversion loses integer precision: 'long' to 'int' [-Wshorten-64-to-32]
            NIL_P(input) ? 0 : RSTRING_LEN(input));
                               ^~~~~~~~~~~~~~~~~~
/usr/home/user/.rvm/rubies/ruby-2.4.5/include/ruby-2.4.0/ruby/ruby.h:980:6: note: expanded from macro 'RSTRING_LEN'
     RSTRING_EMBED_LEN(str) : \
     ^~~~~~~~~~~~~~~~~~~~~~
/usr/home/user/.rvm/rubies/ruby-2.4.5/include/ruby-2.4.0/ruby/ruby.h:976:6: note: expanded from macro 'RSTRING_EMBED_LEN'
     (long)((RBASIC(str)->flags >> RSTRING_EMBED_LEN_SHIFT) & \
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
bcrypt_ext.c:17:25: warning: implicit conversion loses integer precision: 'long' to 'int' [-Wshorten-64-to-32]
            NIL_P(input) ? 0 : RSTRING_LEN(input));
                               ^~~~~~~~~~~~~~~~~~
/usr/home/user/.rvm/rubies/ruby-2.4.5/include/ruby-2.4.0/ruby/ruby.h:981:28: note: expanded from macro 'RSTRING_LEN'
     RSTRING(str)->as.heap.len)
     ~~~~~~~~~~~~~~~~~~~~~~^~~
2 warnings and 1 error generated.
*** Error code 1

Stop.
make: stopped in /usr/home/user/.rvm/gems/ruby-2.4.5/gems/bcrypt-3.1.12/ext/mri

make failed, exit code 1

Gem files will remain installed in /home/user/.rvm/gems/ruby-2.4.5/gems/bcrypt-3.1.12 for inspection.
Results logged to /home/user/.rvm/gems/ruby-2.4.5/extensions/x86_64-freebsd-12/2.4.0/bcrypt-3.1.12/gem_make.out