define a more generic LIBFUZZER = ... in Makefile.in

The current value

LIBFUZZER = /home/nickm/build/libfuzz/libFuzzer.a

isn't portable enough IMO. BTW if I configure the sources (with clang 3.9.1):

CC="clang" ./configure --config-cache --enable-expensive-hardening --enable-libfuzzer

then I run during make into

$ make
make  all-am
make[1]: Entering directory '/home/tfoerste/devel/tor'
  AR       src/ext/keccak-tiny/libkeccak-tiny.a
  AR       src/trunnel/libor-trunnel.a
  CC       src/ext/trunnel/src_trunnel_libor_trunnel_testing_a-trunnel.o
clang-3.9: error: unsupported argument 'trace-pc-guard' to option 'fsanitize-coverage='
clang-3.9: error: unsupported argument 'trace-div' to option 'fsanitize-coverage='
make[1]: *** [Makefile:7704: src/ext/trunnel/src_trunnel_libor_trunnel_testing_a-trunnel.o] Error 1
make[1]: Leaving directory '/home/tfoerste/devel/tor'
make: *** [Makefile:3044: all] Error 2

at a stable (hardened) Gentoo Linux.