Export only symbols intended for external use

[pcolberg]

From nm -gDC libalteracl.so on Linux, or dumpbin /exports alteracl.dll on Windows, the runtime shared library exports many symbols that are not intended for external use, e.g., clCreateProgramWithBinaryAndProgramDeviceIntelFPGA() whose address should instead be looked up via clGetExtensionFunctionAddress().

This is a regression from previous versions of the runtime library shipped with the
Intel® FPGA SDK for OpenCL™ Software Technology, which exports only symbols intended for external use.