Detect support for compiler symbol visibility flags and apply those
according to CURL_HIDDEN_SYMBOLS option.
It should work true to the autotools build except it tries to unhide
symbols on Windows when requested and prints warning if it fails.
Ref: #981 (comment)
Reported-by: Daniel Stenberg
This only excludes building unit tests from default build ( 'all' Make
target or "Build Solution" in VisualStudio). The projects and Make
targets will still be generated and shown in supporting IDEs.
Reported-by: Randy Armstrong