TODO: write a generic framework for that stuff

[Test] Add test for composites with symbol groups

Test build with clang

It turned out that clang significantly slower than gcc on rspamd code
(at least with -O0). Switch off torch to decrease build time.

gboolean support was removed in commit 91287bb840.

Build clang plugin and use it to check rspamd printf-like functions
(rspamd_printf, rspamd_fprintf, rspamd_default_log_function, e.t.c).

llvm-config --cxxflags shows CFLAGS used to compile clang. If clang
installed from package and package was build using GCC then it will
contain GCC specific flags. In case of clang installed as RPM packge
-specs= flags also added. There is not point to use them to
compile plugin. For now hardcode flags in clang-plugin/CMakeLists.txt

Add -O2 - we want plugin to work fast even if we to compiling rspamd
code with -O0.

[Test] Add composites test

Stop linking with -lnsl on Linux

As I can see this variable is not used by rspamd build system. Remove it
because setting it causes a lot of warning with
-DENABLE_CLANG_PLUGIN=ON - PREFIX variable is not documented but used by
cmake internally.

Compilation with -O2/-O3 is slow.

As far as I know on Linux libnsl is used only for NIS/NIS+ support.
It is highly unlikey that Rspamd users use NIS in 2018.

Keep linking with libnsl on Solaris - there it is likely needed even if
NIS is not used.

Speedup lua coverage collecting for functional test

luacov-coveralls merge mode (-j flag) was created to join reports
containing coverage for different source files (e.g. C and Lua code).
Coverage for the same file in two report is not merged, instead one
source file is added several times to source_files array in JSON. As
a result if we use luacov-coveralls -j on report for same source files
it ends up spending a lot of time on parsing and dumping big JSON files.

This change reduces functional test time from 7+ minutes to 4+ minutes.