I encounter this issue with ruby version 2.1.10, OSX El Capitan and the description of error as below:
Installing bcrypt-ruby 3.1.2 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.
checking for ruby/util.h... yes
creating Makefile
make "DESTDIR=" clean
make "DESTDIR="
compiling bcrypt_ext.c
compiling crypt.c
compiling crypt_blowfish.c
compiling crypt_gensalt.c
compiling wrapper.c
linking shared-object bcrypt_ext.bundle
ld: warning: directory not found for option '- L/Users/travis/.sm/pkg/active/lib'
make "DESTDIR=" install
make: /usr/local/bin/gmkdir: No such file or directory
make: *** [.RUBYARCHDIR.time] Error 1
make install failed, exit code 2