~$ gem install xmlsec
Building native extensions.  This could take a while...
ERROR:  Error installing xmlsec:
    ERROR: Failed to build gem native extension.

    /usr/local/bin/ruby extconf.rb
checking for xmlSecDSigCtxCreate() in -lxmlsec1... yes
checking for xmlsec/version.h... yes
creating Makefile

make "DESTDIR="
compiling xmlsec_ext.c
xmlsec_ext.c: In function ‘Init_xmlsec_ext’:
xmlsec_ext.c:53: error: ‘openssl’ undeclared (first use in this function)
xmlsec_ext.c:53: error: (Each undeclared identifier is reported only once
xmlsec_ext.c:53: error: for each function it appears in.)
make: *** [xmlsec_ext.o] Error 1

Gem files will remain installed in /usr/local/lib/ruby/gems/2.0.0/gems/xmlsec-0.0.6 for inspection.
Results logged to /usr/local/lib/ruby/gems/2.0.0/gems/xmlsec-0.0.6/ext/xmlsec/gem_make.out