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

Index: posix.mak
--- posix.mak.orig
+++ posix.mak
@@ -1,42 +1,8 @@
-DMD_DIR = ../dmd
-BUILD = release
-DMD = $(DMD_DIR)/generated/$(OS)/$(BUILD)/$(MODEL)/dmd
-CC = gcc
-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)/src/osmodel.mak
-
-# Build folder for all binaries
-GENERATED = generated
-ROOT = $(GENERATED)/$(OS)/$(MODEL)
-
-# Set DRUNTIME name and full path
-ifeq (,$(findstring win,$(OS)))
-	DRUNTIME = $(DRUNTIME_PATH)/lib/libdruntime-$(OS)$(MODEL).a
-	DRUNTIMESO = $(DRUNTIME_PATH)/lib/libdruntime-$(OS)$(MODEL)so.a
-else
-	DRUNTIME = $(DRUNTIME_PATH)/lib/druntime.lib
-endif
-
-# Set PHOBOS name and full path
-ifeq (,$(findstring win,$(OS)))
-	PHOBOS = $(PHOBOS_PATH)/generated/$(OS)/$(BUILD)/$(MODEL)/libphobos2.a
-	PHOBOSSO = $(PHOBOS_PATH)/generated/$(OS)/$(BUILD)/$(MODEL)/libphobos2.so
-endif
-
 # default to warnings and deprecations as errors, override via e.g. make -f posix.mak WARNINGS=-wi
 WARNINGS = -w -de
-# default include/link paths, override by setting DFLAGS (e.g. make -f posix.mak DFLAGS=-I/foo)
-DFLAGS = -I$(DRUNTIME_PATH)/import -I$(PHOBOS_PATH) \
-		 -L-L$(PHOBOS_PATH)/generated/$(OS)/$(BUILD)/$(MODEL) $(MODEL_FLAG) -fPIC -preview=dip1000
+DFLAGS = -O -release -inline
 DFLAGS += $(WARNINGS)
 
 # Default DUB flags (DUB uses a different architecture format)