Ignore ldconfig failures when installing on Linux (#346)

* Ignore ldconfig failures when installing on Linux

Currently `make install PREFIX=$HOME/lib/themis` fails on Linux
when run without superuser privileges. Installation to non-system
directories should not require superuser privileges. This does not
work because of an unconditional `ldconfig` invocation.

Linux systems generally require an `ldconfig` invocation after
installing shared libraries in order to refresh ld.so's library
cache. Themis' "make install" target does that unconditionally
when building on Linux. It's not necessary for static libraries
or when installing into non-system locations that require special
configuration of the dynamic loader (rpath, LD_LIBRATY_PATH, etc.)

Note that `ldconfig` requires superuser privileges as it writes
multiple files in /etc. However, "make install" is not always run
with superuser privileges.

There are two basic use-cases for installing Themis:

- Installation into the system directory

  Typically this is used when packaging Themis library by itself.
  This installation requires superuser privileges anyway to copy
  the libraries into system locations. Usually shared libraries
  are installed together with static libraries.

  This is typically invoked as `sudo make install`

- Installation into a temporary directory

  Typically this is used to build a local copy of Themis for use
  inside another project. Themis is installed into some non-system
  location by setting the PREFIX variable accordingly. Usually
  only static libraries are used. No superuser privileges needed.

  This is typically invoked as `make install PREFIX=$somewhere`

The first use-case works fine now, but the second one is broken
by `ldconfig` failing the build without superuser privileges.

Let's fix it in a simple and lazy way by ignoring any errors caused
by `ldconfig`. (They are still printed out to stderr though.)

Strictly speaking, we should invoke `ldconfig` only after installing
shared libraries. And it's only needed when installing into system
locations. However, it's too complex for us to analyze EUID and
PREFIX values. And it's certainly easier for the users to simply
run "make install" all the time. Though, it's definitely possible
to add a new target which will not call `ldconfig`:

    install_static: install_soter_headers install_themis_headers \
                    install_static_libs install_pkgconfig

    install: install_static install_shared_libs
    ifdef IS_LINUX
            @ldconfig
    endif

However, there may be cases when shared libraries need to be
installed into non-system locations as well. So it should be more
easy to simply ignore `ldconfig` failures.

* Ignore ldconfig failures only for non-root users

Actually, do check that we are running as root by looking at our EUID.
If we're a non-root user then ignore ldconfig failures. Otherwise exit
with the status code returned by ldconfig and propagate the error.

Makefile

@@ -365,7 +365,7 @@ install_pkgconfig: err all make_install_dirs
 
 install: install_soter_headers install_themis_headers install_static_libs install_shared_libs install_pkgconfig
 ifdef IS_LINUX
-	@ldconfig
+	@ldconfig || (status=$$?; if [ $$(id -u) = "0" ]; then exit $$status; else exit 0; fi)
 endif
 
 get_version: