The stripcredentials unittest fails to compile on platforms without
xattr support, for example the Solaris member in the buildfarm which
fails with the following:
CC unit1621-unit1621.o
CC ../libtest/unit1621-first.o
CCLD unit1621
Undefined first referenced
symbol in file
stripcredentials unit1621-unit1621.o
goto problem 2
ld: fatal: symbol referencing errors. No output written to .libs/unit1621
collect2: error: ld returned 1 exit status
gmake[2]: *** [Makefile:996: unit1621] Error 1
Fix by excluding the test on such platforms by using the reverse
logic from where stripcredentials() is defined.
Closes#3759
Reviewed-by: Daniel Stenberg <daniel@haxx.se>
