0. Install Visual Studio 2017 (Community Edtion or whatever), select "Windows XP support".

1. Download and unpack OpenSSL 1.0.x sources and prepare build. Mind the paths.

"C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build\vcvarsall.bat" x86
set PATH=C:\Jom;C:\Program Files (x86)\Microsoft SDKs\Windows\v7.1A\Bin;%PATH%
set INCLUDE=C:\Program Files (x86)\Microsoft SDKs\Windows\v7.1A\Include;%INCLUDE%
set LIB=C:\Program Files (x86)\Microsoft SDKs\Windows\v7.1A\Lib;%LIB%
set CL=/D_USING_V110_SDK71_

2. Compile OpenSSL.

cd C:\OpenSSL\Src\1.0.2q
perl Configure VC-WIN32 no-asm no-async --prefix=C:\OpenSSL\1.0.2q --openssldir=C:\OpenSSL\1.0.2q
ms\do_nt
notepad ms\nt.mak => add -D_USING_V110_SDK71_ -I"C:\Program Files (x86)\Microsoft SDKs\Windows\v7.1A\Include" -DPSAPI_VERSION=1 to CFLAG
nmake -f ms\nt.mak install

3. Compile Qt.

cd C:\Qt\5.6.3\Src
set INCLUDE=C:\OpenSSL\1.0.2q\include;%INCLUDE%
set LIB=C:\OpenSSL\1.0.2q\lib;%LIB%
set OPENSSL_LIBS="-LC:/OpenSSL/1.0.2q/lib -llibeay32 -lssleay32 -lgdi32"
configure -prefix C:\Qt\5.6.3\msvc2017-static -static -static-runtime -target xp -platform win32-msvc2017 -debug-and-release -qmake -opensource -nomake examples -no-compile-examples -nomake tests -qt-pcre -no-icu -no-sql-sqlite -no-nis -no-cups -no-iconv -no-dbus -qt-zlib -qt-libpng -opengl desktop -no-directwrite -I "C:\Program Files (x86)\Microsoft SDKs\Windows\v7.1A\Include" -L "C:\Program Files (x86)\Microsoft SDKs\Windows\v7.1A\Lib" -I C:\OpenSSL\1.0.2q\include -L C:\OpenSSL\1.0.2q\lib -openssl -openssl-linked OPENSSL_LIBS="-lssleay32 -llibeay32 -lgdi32"
jom
jom install