Updated to support RHEL7 (and CentOS7)

[juszak]

These changes add some notes for building on RHEL7/CentOS7, updates CMakeLists.txt to include CMAKE_THREAD_LIBS_INIT, and updates the build_dependencies.sh script to build cpprestsdk on RHEL7/CentOS7.

I tested this using GCC 7.3, installed like this...

sudo yum-config-manager --enable rhel-server-rhscl-7-rpms
sudo yum -y install devtoolset-7
echo "source scl_source enable devtoolset-7 &> /dev/null" >> ~/.bashrc 
source ~/.bashrc

The default GCC version on RHEL7 is pretty old; GCC 4.8. I'm not sure how many folks would still be trying that version.

Anyway, I used the micro-service app to figure out a few things with cpprestsdk and it was a big help. I thought I'd post back what I did to get it running on RHEL7.