Put NIF build products under the _build directory

The Makefile was storing build products in the source directory. This
causes issues on Nerves when switching target processors since the NIF
won't be built for the right architecture and the Makefile won't know to
rebuild it. Building under the _build directory fixes this since there's
a different subdirectory for every target processor (this uses the mix
targets feature).

This also addresses a source of confusion on Apple M1 Macs where you can
switch between Intel and M1 modes. Since mix targets aren't used, you
still need to be aware that you need to build clean when switching
modes. It seems less confusing that building clean involves deleting
files from `_build` rather both `_build` and `deps`.

Notes about this change:

1. The `Makefile` now depends on environment variables set by
   `elixir_make` for the location of the `_build` directory and `erl_nif.h`.
   This means that calling `make` to only build the NIF won't work. See
   next commit.
2. This change works with Nerves. The Nerves infrastructure uses the
   same `elixir_make` environment variables, so the `erl_nif.h` include
   directory will come from Erlang source directory that matches the
   Nerves target. This didn't happen before, but `erl_nif.h` must not
   change enough between Erlang versions to matter. (i.e., it was lucky)

Makefile

@@ -1,10 +1,10 @@
 CFLAGS = -g -O3 -Wall -Wno-format-truncation
 
-ERLANG_PATH = $(shell erl -eval 'io:format("~s", [lists:concat([code:root_dir(), "/erts-", erlang:system_info(version), "/include"])])' -s init stop -noshell)
-CFLAGS += -I"$(ERLANG_PATH)"
+CFLAGS += -I"$(ERTS_INCLUDE_DIR)"
 CFLAGS += -Ic_src
 
-LIB_NAME = priv/bcrypt_nif.so
+PRIV_DIR = $(MIX_APP_PATH)/priv
+LIB_NAME = $(PRIV_DIR)/bcrypt_nif.so
 ifneq ($(CROSSCOMPILE),)
     # crosscompiling
     CFLAGS += -fPIC
@@ -23,12 +23,14 @@ NIF_SRC=\
 	c_src/bcrypt_nif.c\
 	c_src/blowfish.c
 
-all: $(LIB_NAME)
+all: $(PRIV_DIR) $(LIB_NAME)
 
 $(LIB_NAME): $(NIF_SRC)
-	mkdir -p priv
 	$(CC) $(CFLAGS) -shared $(LDFLAGS) $^ -o $@
 
+$(PRIV_DIR):
+	mkdir -p $@
+
 clean:
 	rm -f $(LIB_NAME)