BUILDING_LIBCURL and CURL_STATICLIB are no longer defined in curl_config.h,
configure will generate appropriate conditionals so that mentioned symbols
get defined and used in Makefiles at compilation time
When configure --enable-debug has been used, all files in lib/ are now
built twice and a separate static library crafted for unit-testing will
be linked. The unit tests in the tests/unit subdir will use that
library.