BUILD: libmariadb/plugins/auth/ have_C__Wl___as_needed is set incorrectly

The following line in CMakeLists.txt is erroneous:

CHECK_C_COMPILER_FLAG(-Wl,--as-needed have_C__Wl___as_needed)

The problem is that it uses CHECK_C_COMPILER_FLAG() to test for a linker flag, which can result in false positives. Need to use CHECK_LINKER_FLAG() instead:

CHECK_LINKER_FLAG(C --as-needed have_C__Wl___as_needed)

This is a regression from 10.6.10.