Error installing bcrypt-ruby

I have run the command: gem install bcrypt '3.1.7'.

Unfortunately the gem fails to install. Instead I get the following output:

 Fetching: bcrypt-3.1.7.gem (100%)
Building native extensions.  This could take a while...
ERROR:  Error installing bcrypt-ruby:
        ERROR: Failed to build gem native extension.

    /usr/local/rvm/rubies/ruby-1.8.7-head/bin/ruby extconf.rb
checking for ruby/util.h... yes
creating Makefile

make
gcc -I. -I/usr/local/rvm/rubies/ruby-1.8.7-head/lib/ruby/1.8/x86_64-linux -I/usr/local/rvm/rubies/ruby-1.8.7-head/lib/ruby/1.8/x86_64-linux -I. -DHAVE_RUBY_UTIL_H    -fPIC -O2 -fno-tree
-dce -fno-optimize-sibling-calls  -fPIC   -c crypt.c
gcc -I. -I/usr/local/rvm/rubies/ruby-1.8.7-head/lib/ruby/1.8/x86_64-linux -I/usr/local/rvm/rubies/ruby-1.8.7-head/lib/ruby/1.8/x86_64-linux -I. -DHAVE_RUBY_UTIL_H    -fPIC -O2 -fno-tree
-dce -fno-optimize-sibling-calls  -fPIC   -c crypt_gensalt.c
gcc -I. -I/usr/local/rvm/rubies/ruby-1.8.7-head/lib/ruby/1.8/x86_64-linux -I/usr/local/rvm/rubies/ruby-1.8.7-head/lib/ruby/1.8/x86_64-linux -I. -DHAVE_RUBY_UTIL_H    -fPIC -O2 -fno-tree
-dce -fno-optimize-sibling-calls  -fPIC   -c crypt_blowfish.c
gcc -I. -I/usr/local/rvm/rubies/ruby-1.8.7-head/lib/ruby/1.8/x86_64-linux -I/usr/local/rvm/rubies/ruby-1.8.7-head/lib/ruby/1.8/x86_64-linux -I. -DHAVE_RUBY_UTIL_H    -fPIC -O2 -fno-tree
-dce -fno-optimize-sibling-calls  -fPIC   -c bcrypt_ext.c
gcc -I. -I/usr/local/rvm/rubies/ruby-1.8.7-head/lib/ruby/1.8/x86_64-linux -I/usr/local/rvm/rubies/ruby-1.8.7-head/lib/ruby/1.8/x86_64-linux -I. -DHAVE_RUBY_UTIL_H    -fPIC -O2 -fno-tree
-dce -fno-optimize-sibling-calls  -fPIC   -c wrapper.c
In file included from /usr/include/ruby/util.h:22:0,
                 from wrapper.c:28:
/usr/include/ruby/defines.h:71:7: error: conflicting types for ‘ruby_xmalloc’
 void *xmalloc(size_t);
       ^
In file included from /usr/local/rvm/rubies/ruby-1.8.7-head/lib/ruby/1.8/x86_64-linux/ruby.h:37:0,
                 from wrapper.c:26:
/usr/local/rvm/rubies/ruby-1.8.7-head/lib/ruby/1.8/x86_64-linux/defines.h:49:7: note: previous declaration of ‘ruby_xmalloc’ was here
 void *xmalloc _((long));
       ^
In file included from /usr/include/ruby/util.h:22:0,
                 from wrapper.c:28:
/usr/include/ruby/defines.h:73:7: error: conflicting types for ‘ruby_xcalloc’
 void *xcalloc(size_t,size_t);
       ^
In file included from /usr/local/rvm/rubies/ruby-1.8.7-head/lib/ruby/1.8/x86_64-linux/ruby.h:37:0,
                 from wrapper.c:26:
/usr/local/rvm/rubies/ruby-1.8.7-head/lib/ruby/1.8/x86_64-linux/defines.h:50:7: note: previous declaration of ‘ruby_xcalloc’ was here
 void *xcalloc _((long,long));
       ^
In file included from /usr/include/ruby/util.h:22:0,
                 from wrapper.c:28:
/usr/include/ruby/defines.h:74:7: error: conflicting types for ‘ruby_xrealloc’
 void *xrealloc(void*,size_t);
       ^
In file included from /usr/local/rvm/rubies/ruby-1.8.7-head/lib/ruby/1.8/x86_64-linux/ruby.h:37:0,
                 from wrapper.c:26:
/usr/local/rvm/rubies/ruby-1.8.7-head/lib/ruby/1.8/x86_64-linux/defines.h:51:7: note: previous declaration of ‘ruby_xrealloc’ was here
 void *xrealloc _((void*,long));
       ^
make: *** [wrapper.o] Error 1


Gem files will remain installed in /usr/local/rvm/gems/ruby-1.8.7-head/gems/bcrypt-3.1.7 for inspection.
Results logged to /usr/local/rvm/gems/ruby-1.8.7-head/gems/bcrypt-3.1.7/ext/mri/gem_make.out
ERROR:  Could not find a valid gem '3.1.5' (>= 0) in any repository

This is basically a rather spurious outpouring of problems from 'make' failing to complete. I have found a lot of people have had problems installing bcrypt but none of the suggestions they use solve this particular problem.

How can I solve this problem?