Do not look for rustc

You need cargo, not rustc, and cargo requires rustc,
so checking for rustc is incorrect. And the user will
know that she needs cargo when the command cannot be
find, so why look for it and add extra dependenices
just for that.

Signed-off-by: Mattias Andrée <maandree@kth.se>

Makefile

@@ -3,7 +3,6 @@ PREFIX ?= /usr/local
 BUILD = target/release/exa
 
 $(BUILD):
-	@which rustc > /dev/null || { echo "exa requires Rust to compile. For installation instructions, please visit http://rust-lang.org/"; exit 1; }
 	if test -n "$$(echo "$$CC" | cut -d \  -f 1)"; then \
 	    env CC="$$(echo "$$CC" | cut -d \  -f 1)" cargo build --release; \
 	else\
@@ -13,7 +12,6 @@ $(BUILD):
 build: $(BUILD)
 
 build-no-git:
-	@which rustc > /dev/null || { echo "exa requires Rust to compile. For installation instructions, please visit http://rust-lang.org/"; exit 1; }
 	if test -n "$$(echo "$$CC" | cut -d \  -f 1)"; then \
 	    env CC="$$(echo "$$CC" | cut -d \  -f 1)" cargo build --release --no-default-features; \
 	else\