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
Merge pull request #459 from tkelman/symbol-rename

add SYMBOLPREFIX and SYMBOLSUFFIX makefile options