Added checks of plugin support in underlying Binutils.

* compilers.jam: Renamed `get-ar`, `get-ranlib` and `get-nm` rules to
                 `get-ar-command-substitution`,
                 `get-ranlib-command-substitution` and
                 `get-nm-command-substitution`, respectively. If the
                 underlying Binutils tools (`ar`, `ranlib` and `nm`) for
                 `gcc-ar`, `gcc-ranlib` and `gcc-nm` wrapper commands do not
                 have plugin support, the wrapper commands do not work.
                 Therefore, the renamed rules provide command substitution
                 to produce suitable values for `AR`, `RANLIB`, `NM`,
                 `AR_FOR_TARGET`, `RANLIB_FOR_TARGET` and `NM_FOR_TARGET`
                 environment variables according to plugin support in
                 the underlying Binutils tools.
* binutils/jamfile: Replaced invocations of `get-ar`, `get-ranlib` and
                    `get-nm` rules with ones of
                    `get-ar-command-substitution`,
                    `get-ranlib-command-substitution` and
                    `get-nm-command-substitution`, respectively.
* clang/jamfile: Likewise.
* cloog/jamfile: Likewise.
* gdb/jamfile: Likewise.
* gmp/jamfile: Likewise.
* icu4c/jamfile: Likewise.
* isl/jamfile: Likewise.
* mpc/jamfile: Likewise.
* mpfr/jamfile: Likewise.
* opencv/jamfile: Likewise.
* openmpi/jamfile: Likewise.
* ppl/jamfile: Likewise.
* valgrind/jamfile: Likewise.
Merge pull request #111 from Cryolite/fix/gccAr

Added checks of plugin support in underlying Binutils.