Current libudev-zero source code doesn't have license information in source files.
This patch add license information to source files based on SPDX standard.
Currently, "-soname" set at "libudev.so.1", but building library file name was "libudev.so".
Typical shared library structure as a follow;
libudev.so -> libudev.so.1
libudev.so.1
Current installation is reversed.
This patch fix this.
Ubuntu/Debian style file placement are ;
No.1 libudev.so and libudev.so.1 in /lib or /lib/arch/
No.2 libudev.h in /usr/include
No.3 libudev.pc in /usr/lib/pkgconfig or /usr/lib/arch/pkgconfig
Current Makefile can install No.1 and No.2 pattern by
"make PREFIX=/usr LIBDIR=/lib INCLUDEDIR=/usr/include install",
but it can't install No.3 pattern.
This patch added PKGCONFIGDIR environment variable to achieve all install pattern by
"make PREFIX=/usr LIBDIR=/lib INCLUDEDIR=/usr/include PKGCONFIGDIR=/usr/lib install"
