Use our own compiler instead of cloning a new one.
Set proper compiler flags for the build.

Index: Makefile
--- Makefile.orig
+++ Makefile
@@ -1,32 +1,9 @@
-DMD_DIR = ../dmd
-BUILD = release
-DMD = $(DMD_DIR)/generated/$(OS)/$(BUILD)/$(MODEL)/dmd
-INSTALL_DIR = ../install
-DRUNTIME_PATH = ../dmd/druntime
-PHOBOS_PATH = ../phobos
-DUB=dub
+ROOT = .
 
-WITH_DOC = no
-DOC = ../dlang.org
-
-# Load operating system $(OS) (e.g. linux, osx, ...) and $(MODEL) (e.g. 32, 64) detection Makefile from dmd
-$(shell [ ! -d $(DMD_DIR) ] && git clone --depth=1 https://github.com/dlang/dmd $(DMD_DIR))
-include $(DMD_DIR)/compiler/src/osmodel.mak
-
-ifeq (windows,$(OS))
-    DOTEXE:=.exe
-else
-    DOTEXE:=
-endif
-
-# Build folder for all binaries
-GENERATED = generated
-ROOT = $(GENERATED)/$(OS)/$(MODEL)
-
 # default to warnings and deprecations as errors, override via e.g. make WARNINGS=-wi
 WARNINGS = -w -de
 # default flags, override by setting DFLAGS (e.g. make DFLAGS=-O)
-DFLAGS = $(MODEL_FLAG) $(if $(findstring windows,$(OS)),,-fPIC) -preview=dip1000
+DFLAGS = -O -release -inline -preview=dip1000
 DFLAGS += $(WARNINGS)
 
 # Default DUB flags (DUB uses a different architecture format)
