add SYMBOLPREFIX and SYMBOLSUFFIX makefile options

for adding a prefix or suffix to all exported symbol names in the shared library
Useful to avoid conflicts with other BLAS libraries, especially when using
64 bit integer interfaces in OpenBLAS

Note that since OSX does not have the objcopy utility, setting these options
to non-empty values on Mac requires the objconv tool, available (GPL license)
from http://www.agner.org/optimize/#objconv
(It is only run as a command-line application to change the symbol names at OpenBLAS-build-time so I don't think the GPL license is a problem, and if the options are not set then objconv is not needed.)

Refer to https://github.com/JuliaLang/julia/pull/8734 and https://github.com/JuliaLang/julia/pull/4923 for background motivation.