change LD in src_install with qt4 useflag, to fix a bug akin to #230 on s-o-g

I tried to update pari to 2.5.4-r1 and I've come across this error:

mkdir -p "/var/tmp/portage/sci-mathematics/pari-2.5.4-r1/image//usr/bin"
rm -f "/var/tmp/portage/sci-mathematics/pari-2.5.4-r1/image//usr/bin"/gp-2.5 "/var/tmp/portage/sci-mathematics/pari-2.5.4-r1/image//usr/bin"/gp
../config/install -m 644 ../examples/contfrac.gp "/var/tmp/portage/sci-mathematics/pari-2.5.4-r1/image//usr/share/pari"/examples
x86_64-pc-linux-gnu-gcc  -o "/var/tmp/portage/sci-mathematics/pari-2.5.4-r1/image//usr/bin"/gp-2.5 -L"/var/tmp/portage/sci-mathematics/pari-2.5.4-r1/image//usr/lib64" -O3 -Wall -fno-strict-aliasing -fomit-frame-pointer  -march=native -O2 -pipe  -Wl,--export-dynamic -Wl,--hash-style=gnu -Wl,-O1 -Wl,--as-needed -Wl,--hash-style=gnu -Wl,--sort-common gp.o gp_init.o gp_rl.o highlvl.o whatnow.o plotport.o plotQt4.o -Wl,-rpath,"/usr/lib64":/usr/lib64:/usr/lib -lpari -L/usr/lib -lreadline -L/usr/lib64/qt4 -lQtGui -lQtCore   -lm -ldl
../config/install -m 644 ../examples/lucas.gp    "/var/tmp/portage/sci-mathematics/pari-2.5.4-r1/image//usr/share/pari"/examples
../config/install -m 644 ../examples/extgcd.c    "/var/tmp/portage/sci-mathematics/pari-2.5.4-r1/image//usr/share/pari"/examples
../config/install -m 644 ../examples/rho.gp      "/var/tmp/portage/sci-mathematics/pari-2.5.4-r1/image//usr/share/pari"/examples
../config/install -m 644 ../examples/squfof.gp   "/var/tmp/portage/sci-mathematics/pari-2.5.4-r1/image//usr/share/pari"/examples
../config/install -m 644 ../examples/taylor.gp   "/var/tmp/portage/sci-mathematics/pari-2.5.4-r1/image//usr/share/pari"/examples
/usr/lib/gcc/x86_64-pc-linux-gnu/4.7.2/../../../../x86_64-pc-linux-gnu/bin/ld: plotQt4.o: undefined reference to symbol  «__cxa_rethrow@@CXXABI_1.3»
/usr/lib/gcc/x86_64-pc-linux-gnu/4.7.2/../../../../x86_64-pc-linux-gnu/bin/ld: note: «__cxa_rethrow@@CXXABI_1.3» is defined in DSO /usr/lib/gcc/x86_64-pc-linux-gnu/4.7.2/libstdc++.so.6 so try adding it to the linker command line
/usr/lib/gcc/x86_64-pc-linux-gnu/4.7.2/libstdc++.so.6: could not read symbols: Invalid operation
collect2: error: ld returned 1 exit status
make[1]: *** [install-bin-dyn] Error 1

This is like the error found in issue #230, but this time in the install phase. There is a call to $(LD) in the target "install-bin-dyn" and LD is set to gcc. I fixed it by exporting LD=g++ in the src_install phase.