jeanpat@dip4fish ~]$ whereis gcc-4.9
gcc-4: /usr/bin/gcc-4.9
[jeanpat@dip4fish ~]$ whereis g++-4.9
g++-4: /usr/bin/g++-4.9
[jeanpat@dip4fish ~]$ whereis g++
g++: /usr/bin/g++ /usr/share/man/man1/g++.1.gz
[jeanpat@dip4fish ~]$ whereis gcc
gcc: /usr/bin/gcc /usr/lib/gcc /usr/share/man/man1/gcc.1.gz /usr/share/info/gcc.info.gz

jeanpat@dip4fish ~]$ export CC=/usr/bin/gcc-4.9
[jeanpat@dip4fish ~]$ export CXX=/usr/bin/g++-4.9

[jeanpat@dip4fish ~]$ gcc --version
gcc (GCC) 6.1.1 20160602

$ echo $LD_LIBRARY_PATH
:/opt/cuda/lib64: