'gem install ffi' failed on Mac OS X Yosemite 10.10.5 - ruby-on-rails

Question

I'm trying to set up a RoR environment under RVM on Mac OS X Yosemite 10.10.5 on my macbook pro.
The ruby version that I need to install is 1.9.3-p194 because it's required for my software development project.
So far, I have the following software installed on my Macbook:
RVM
ruby 1.9.3-p194
xcode v6.4 and the respective version of command line tools
Mac OS X version: 10.10.5 Yosemite
It seems I'm running into a problem that the ffi gem (an independency of my project) cannot be built successfully.
I get the following error whenever I try to install the gem via the command "gem install ffi -v '1.9.3'"
MacBook-Pro:demo-project apple$ gem install ffi -v '1.9.3'
Building native extensions. This could take a while...
ERROR: Error installing ffi:
ERROR: Failed to build gem native extension.
/Users/apple/.rvm/rubies/ruby-1.9.3-p194/bin/ruby -r ./siteconf20160428-1898-idt325.rb extconf.rb
checking for ffi_call() in -lffi... *** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of
necessary libraries and/or headers. Check the mkmf.log file for more
details. You may need configuration options.
Provided configuration options:
--with-opt-dir
--with-opt-include
--without-opt-include=${opt-dir}/include
--with-opt-lib
--without-opt-lib=${opt-dir}/lib
--with-make-prog
--without-make-prog
--srcdir=.
--curdir
--ruby=/Users/apple/.rvm/rubies/ruby-1.9.3-p194/bin/ruby
--with-ffi_c-dir
--without-ffi_c-dir
--with-ffi_c-include
--without-ffi_c-include=${ffi_c-dir}/include
--with-ffi_c-lib
--without-ffi_c-lib=${ffi_c-dir}/lib
--with-libffi-config
--without-libffi-config
--with-pkg-config
--without-pkg-config
--with-ffilib
--without-ffilib
/Users/apple/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/mkmf.rb:381:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
from /Users/apple/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/mkmf.rb:461:in `try_link0'
from /Users/apple/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/mkmf.rb:476:in `try_link'
from /Users/apple/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/mkmf.rb:619:in `try_func'
from /Users/apple/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/mkmf.rb:845:in `block in have_library'
from /Users/apple/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/mkmf.rb:790:in `block in checking_for'
from /Users/apple/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/mkmf.rb:284:in `block (2 levels) in postpone'
from /Users/apple/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/mkmf.rb:254:in `open'
from /Users/apple/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/mkmf.rb:284:in `block in postpone'
from /Users/apple/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/mkmf.rb:254:in `open'
from /Users/apple/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/mkmf.rb:280:in `postpone'
from /Users/apple/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/mkmf.rb:789:in `checking_for'
from /Users/apple/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/mkmf.rb:840:in `have_library'
from extconf.rb:20:in `<main>'
extconf failed, exit code 1
Gem files will remain installed in /Users/apple/.rvm/gems/ruby-1.9.3-p194/gems/ffi-1.9.3 for inspection.
Results logged to /Users/apple/.rvm/gems/ruby-1.9.3-p194/extensions/x86_64-darwin-14/1.9.1/ffi-1.9.3/gem_make.out
I said I have the xcode command line tools installed, but notice from the given error below that it gave me a hint "You have to install development tools first".
Didn't I have it installed already? I was asking myself out of curiousity.
Then I googled and found out the way how to check if I have xcode command line tools installed. Then I launched the terminal and issued that command as illustrated below, and the result did confirm that the piece of software was already installed.
MacBook-Pro:demo-project apple$ xcode-select -p
/Applications/Xcode.app/Contents/Developer
Here is also my gcc version:
MacBook-Pro:demo-project apple$ gcc -v
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)
Target: x86_64-apple-darwin14.5.0
Thread model: posix
I have searched all over the web and tried many solutions, but, unfortunately, without success.
Any advice would be very much appreciated to get this to work.
Updated:
My mkmf.log
package configuration for libffi
cflags: -I/usr/local/Cellar/libffi/3.0.13/lib/libffi-3.0.13/include
ldflags: -L/usr/local/Cellar/libffi/3.0.13/lib
libs: -lffi
"/usr/local/bin/gcc-4.9 -o conftest -I/Users/apple/.rvm/rubies/ruby-1.9.3-p194/include/ruby-1.9.1/x86_64-darwin14.5.0 -I/Users/apple/.rvm/rubies/ruby-1.9.3-p194/include/ruby-1.9.1/ruby/backward -I/Users/apple/.rvm/rubies/ruby-1.9.3-p194/include/ruby-1.9.1 -I. -I/usr/local/opt/libyaml/include -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE -O3 -ggdb -Wextra -Wno-unused-parameter -Wno-parentheses -Wno-long-long -Wno-missing-field-initializers -Wpointer-arith -Wwrite-strings -Wdeclaration-after-statement -Wimplicit-function-declaration -O3 -I/usr/local/opt/readline/include -I/usr/local/opt/libksba/include -I/usr/local/opt/openssl/include -fno-common -pipe -I/usr/local/Cellar/libffi/3.0.13/lib/libffi-3.0.13/include conftest.c -L. -L/Users/apple/.rvm/rubies/ruby-1.9.3-p194/lib -L/usr/local/opt/libyaml/lib -L. -L/usr/local/opt/readline/lib -L/usr/local/opt/libksba/lib -L/usr/local/opt/openssl/lib -L/usr/local/Cellar/libffi/3.0.13/lib -lffi -lruby.1.9.1 -lpthread -ldl -lobjc "
checked program was:
/* begin */
1: #include "ruby.h"
2:
3: int main() {return 0;}
/* end */