Carry MBedTLS patch that works around CMake 3.18.2 bug.

MBedTLS fails to build with CMake 3.18.2, due to a bug in
CMake 3.18.2 causing failure to find python3. For more information,
please see Mbed-TLS/mbedtls#3690 and
https://gitlab.kitware.com/cmake/cmake/-/issues/21204.

This pull request applies the MBedTLS patch that works around this
issue from Mbed-TLS/mbedtls#3691, which
has been merged into MBedTLS's development branch but not
yet into a tagged release.

Author: Sacha0

deps/mbedtls.mk

@@ -23,6 +23,8 @@ $(SRCCACHE)/$(MBEDTLS_SRC)/source-extracted: $(SRCCACHE)/$(MBEDTLS_SRC).tar.gz
 	$(JLCHECKSUM) $<
 	mkdir -p $(dir $@) && \
 	$(TAR) -C $(dir $@) --strip-components 1 -xf $<
+	# Apply workaround for CMake 3.18.2 bug (https://github.com/ARMmbed/mbedtls/pull/3691).
+	cd $(SRCCACHE)/$(MBEDTLS_SRC) && patch -p1 -f < $(SRCDIR)/patches/mbedtls-cmake-findpy.patch
 	# Force-enable MD4
 	sed "s|//#define MBEDTLS_MD4_C|#define MBEDTLS_MD4_C|" -i $(SRCCACHE)/$(MBEDTLS_SRC)/include/mbedtls/config.h
 	touch -c $(SRCCACHE)/$(MBEDTLS_SRC)/CMakeLists.txt # old target

deps/patches/mbedtls-cmake-findpy.patch

@@ -0,0 +1,23 @@
+diff --git a/CMakeLists.txt b/CMakeLists.txt
+index 8833246..2ed55ed 100644
+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -17,6 +17,18 @@
+ #
+
+ cmake_minimum_required(VERSION 2.6)
++
++# https://cmake.org/cmake/help/latest/policy/CMP0011.html
++# Setting this policy is required in CMake >= 3.18.0, otherwise a warning is generated. The OLD
++# policy setting is deprecated, and will be removed in future versions.
++cmake_policy(SET CMP0011 NEW)
++# https://cmake.org/cmake/help/latest/policy/CMP0012.html
++# Setting the CMP0012 policy to NEW is required for FindPython3 to work with CMake 3.18.2
++# (there is a bug in this particular version), otherwise, setting the CMP0012 policy is required
++# for CMake versions >= 3.18.3 otherwise a deprecated warning is generated. The OLD policy setting
++# is deprecated and will be removed in future versions.
++cmake_policy(SET CMP0012 NEW)
++
+ if(TEST_CPP)
+  project("mbed TLS" C CXX)
+ else()