Introduce multiple improvements to Makefile to make the build more
flexible and support install:
* Support overriding CFLAGS by user but keep required flags in place.
ALL_FLAGS is used in Makefile as recommended in [1].
* Add additional BUILD_SHARED flag to build dynamically linked flavor of
the library. If the flag is set, -fPIC is also passed to make it
possible to build .so.
* Support building in a separate directory provided by OBJDIR variable.
* Add multiple install targets. By default the library itself and libbpf
headers are installed (install target). UAPI headers can be optionally
installed by user.
* All installation paths, including PREFIX, library and include
directories can be overridden. UAPI can be made different from include
directory for libbpf headers. That makes it possible to keep latest
<linux/bpf.h> in a place that doesn't conflict with the one installed
e.g. by kernel-headers package and use it in user's build system.
* Support DESTDIR (see [2]).
* Support overriding LDFLAGS.
* Use utilities such as rm directly as recommended in [3].
* Use compiler and related programs (such as ar) via make variables as
recommended in [3].
* In clean rule remove all possible build artifacts not to rely on passed
options (e.g. if build was done w/ BUILD_SHARED, but clean w/o it).
* Document new build options in README.
[1] https://www.gnu.org/software/make/manual/html_node/Command-Variables.html#Command-Variables
[2] https://www.gnu.org/prep/standards/html_node/DESTDIR.html
[3] https://www.gnu.org/software/make/manual/html_node/Utilities-in-Makefiles.html#Utilities-in-Makefiles
Signed-off-by: Andrey Ignatov <rdna@fb.com>
