diet libc is not correctly detected.

[sasanj]

I am trying to build webfsd using diet, but as you can see in the following output, diet is not detected.

$ make --version
GNU Make 4.2.1
Built for x86_64-pc-linux-gnu
Copyright (C) 1988-2016 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

$ which diet
/usr/bin/diet

$ test -x /usr/bin/diet && echo "yes"
yes

$ make verbose=1
checking for libdir name ... 
run: cc -print-search-dirs | grep -q lib64 && echo lib64 || echo lib
... result is lib

checking for system ... 
run: uname -s | tr 'A-Z' 'a-z'
... result is linux

checking for openssl/ssl.h ... 
run: echo '#include <openssl/ssl.h>' | cc -g -O2 -Wall -Wmissing-prototypes -Wstrict-prototypes -Wpointer-arith -Wunused -DMIMEFILE="/etc/mime.types" -DWEBFS_VERSION="1.21" -D_GNU_SOURCE -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -DNO_SENDFILE -E -
... result is yes

checking for diet ... 
run: which diet
run: test -x 6515bin
... result is no

I got the same result using a GNU Make 4.1, also.