Fix libsystemd detection

[Chocobo1]

1. Use uppercase for prefix parameter SYSTEMD_CFLAGS is used elsewhere so prefix cannot be lowercase. https://autotools.info/pkgconfig/pkg_check_modules.html

2. The module name should be libsystemd Previously it will result in the following compile error when building pppd/auth.c:

   /usr/bin/ld: pppd-auth.o: undefined reference to symbol 'sd_notify@@LIBSYSTEMD_209' /usr/bin/ld: /usr/lib/libsystemd.so.0: error adding symbols: DSO missing from command line

   This is due to missing -lsystemd-daemon flag which is provided by libsystemd-daemon-devel package on Debian or systemd-libs on ArchLinux. And the proper .pc file in the package is libsystemd not systemd. https://stackoverflow.com/a/38303241