Changing build tool for test/random-corpus to CXX instead of CC.

Without this change, GCC complains as follows:
gcc  test/random-corpus.o test/database-test.o notmuch-config.o command-line-arguments.o lib/libnotmuch.a util/libutil.a parse-time-string/libparse-time-string.a -o test/random-corpus -lgmime-2.6 -lgio-2.0 -lgobject-2.0 -lglib-2.0   -Wl,-rpath,/usr/lib -ltalloc   -lxapian
/usr/bin/ld: lib/libnotmuch.a(database.o): undefined reference to symbol '_ZNSs4_Rep10_M_destroyERKSaIcE@@GLIBCXX_3.4'
/usr/bin/ld: note: '_ZNSs4_Rep10_M_destroyERKSaIcE@@GLIBCXX_3.4' is defined in DSO /usr/lib/libstdc++.so.6 so try adding it to the linker command line
/usr/lib/libstdc++.so.6: could not read symbols: Invalid operation
collect2: error: ld returned 1 exit status
make: *** [test/random-corpus] Error 1