Add 'make postgres' Makefile target.

That would build postgres and install it into REPO_ROOT/tmp_install where pageserver
integration tests would be able to find it.

Makefile

@@ -0,0 +1,34 @@
+#
+#   Purpose of this Makefile is to build and install postgres in a local directory
+# so that zenith intergation tests would find pg binaries and support files.
+#
+# 'make postgres' would do following:
+#
+#   1) run out-of-source build of postgres in REPO_ROOT/tmp_install/build directory (I'm reusing
+#  tmp_install path here since it is already present in .gitignore)
+#
+#   2) installs postgres to REPO_ROOT/tmp_install/
+#
+#   This Makefile is integrated with cargo build by the means of build.rs file (special file
+# that cargo build would run if it is present) which will call `make postgres` if there is no
+# tmp_install/bin/postgres binary.
+#
+
+../tmp_install/build/Makefile:
+	mkdir -p ../tmp_install/build && \
+		cd ../tmp_install/build && \
+		../../configure CFLAGS='-O0' --enable-debug --enable-cassert \
+			--enable-depend --with-libxml --prefix=/
+
+pg-configure: ../tmp_install/build/Makefile
+
+# this makefile would set env variables that would interfere with postgres build
+# preventing it from finding autogenerated headers. Hence 'env -i'
+pg-build: pg-configure
+	env -i make -j8 -C ../tmp_install/build
+
+postgres: pg-build
+	mkdir -p ../tmp_install/log
+	env -i make -C ../tmp_install/build \
+		DESTDIR=/Users/stas/code/postgres/tmp_install install \
+		> /Users/stas/code/postgres/tmp_install/log/install.log 2>&1