Add mimalloc as a git submodule

This change adds mimalloc as a git submodule so that `make` doesn't
try to clone mimalloc repo. This change should allow building mold
with the nix package manager, which doesn't provide network access
while building.

This change may conflict with a local `mimalloc` directory. If you
already have checked out mold and `mimalloc` directory exists in the
mold directory, you may need to remove it by `rm -r mimalloc` before
running `make`.

Requested by David Yamnitsky.

Fixes https://github.com/rui314/mold/issues/50

.gitignore

@@ -7,5 +7,4 @@ options.inc
 core
 ld
 oneTBB
-mimalloc
 gentoo

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "mimalloc"]
+	path = mimalloc
+	url = https://github.com/microsoft/mimalloc.git

Makefile

@@ -64,8 +64,7 @@ $(MIMALLOC_LIB): mimalloc/CMakeLists.txt
 	$(MAKE) -C mimalloc/out/release
 
 mimalloc/CMakeLists.txt:
-	git clone https://github.com/microsoft/mimalloc
-	(cd mimalloc; git checkout -q v2.0.1)
+	git submodule update --init --recursive
 
 test tests check: all
 	 $(MAKE) -C test --output-sync --no-print-directory

README.md

@@ -63,7 +63,7 @@ environment, you can build mold by the following commands.
 
 ```
 $ sudo apt-get install build-essential libstdc++-10-dev cmake clang libssl-dev zlib1g-dev libxxhash-dev libtbb-dev git
-$ git clone https://github.com/rui314/mold.git
+$ git clone --recursive https://github.com/rui314/mold.git
 $ cd mold
 $ make
 ```

mimalloc

@@ -0,0 +1 @@
+Subproject commit b19da8e362acae8944c60a40cf5c40c8f5ebeb44