MAKE: create missing directory
MAKE: respect DESTDIR variable

do NOT prepend DESTDIR to PREFIX as they are used seperately in build scripts
MAKE: use simpler syntax
MAKE: use autotools-like variables
MAKE: respect LIBDIR variable

needed for multiarch systems
MAKE: fix install failure caused by partial build
MAKE: fix install failure caused by missing directory
MAKE: don't let "install" depend on "all"

we can even install without any libs built (headers-only)
MAKE: respect system CFLAGS

-g can be an undesired flag and it's better policy
to only add it when environment CFLAGS are empty
MAKE: respect system LDFLAGS

and use LIBS for libraries instead of LDFLAGS
MAKE: respect CFLAGS in linking command as well

there are some CFLAGS which also affect linking, so it's safer
to add it here as well
MAKE: handle SONAME properly

and fix install-rule to make a symlink