common.cmake: provide the CL_LINK_GCC_PLUGIN macro

... for linking GCC plug-ins with Code Listener
common.cmake: specify the entry point when linking GCC plug-ins
cl: build the GCC front-end as a separate library
maint: build gcc-4.8.0 by default
common.cmake: provide the CL_BUILD_GCC_PLUGIN macro

... for building GCC plug-ins with Code Listener
cl/gcc: improve the location validity check

... in order to provide more reliable location info with gcc-4.8.0

Reported by: Daniela Ďuričeková
cl/pointsto_fics: add && and || to the white-list

They are produced by CLSP on pt-1300.c.
cl/tests/data/pt-*.c: fix compile-time warnings
sl/symproc: eliminate a CLSP-specific assertion failure
cl/pointsto: restore a workaround for a USE_AFTER_FREE bug
vra: a Value-Range Analyzer built on top of Code Listener
Makefile: append 'vra' on the list of analyzers
vra: hook unit and overall tests on 'make check'
vra/tests-unit: allow to compile gtest on its own

The compilation failed if gtest/gtest.h was not installed on the system:

make -C gtest
make[1]: Entering directory `/home/kdudka/git/predator/vra/tests-unit/gtest'
g++ -c -pthread -o libgtest.a gtest-all.cc
gtest-all.cc:39:25: fatal error: gtest/gtest.h: No such file or directory
compilation terminated.
make[1]: *** [all] Error 1
make[1]: Leaving directory `/home/kdudka/git/predator/vra/tests-unit/gtest'
make: *** [gtest/libgtest.a] Error 2
vra/tests-overall: make it possible to use system GCC
vra: denote closed intervals by angle brackets
vra: rename LoopFounder -> LoopFinder
vra: minor white-space changes
build-aux: update compatibility patches for old versions of GCC
sl/symexec: do not shadow a param of SymExec::execFnc()