ci: Add the targets/ subdirectory to facilitate cross-platform testing

Considering that a non-trivial amount of libpng code is arch-specific,
we should perform cross-platform builds (with cross-platform toolchains)
and test runs (on emulated architectures) in our routine verification.

The content of ci/targets/ shall consist of target description files,
written in the standard shell language. These files may be source'd as
needed, before running the verification scripts ci/ci_verify_*.sh.

Here is the initial list of target systems:
Android, Cygwin, FreeBSD, Linux, MSDOS, Windows.

And here is the initial list of target architectures:
ARM, MIPS, PowerPC, RISC-V, x86.

ci/targets/windows/ci_env.i686-w64-mingw32.sh

@@ -0,0 +1,21 @@
+# Copyright (c) 2023-2024 Cosmin Truta.
+#
+# Use, modification and distribution are subject to the MIT License.
+# Please see the accompanying file LICENSE_MIT.txt
+#
+# SPDX-License-Identifier: MIT
+
+export CI_TARGET_ARCH=i686
+export CI_TARGET_SYSTEM=mingw32
+
+# The output of `uname -s` on MSYS2 is understandable, and so is
+# CI_TARGET_SYSTEM above, in simplified form. (See also Cygwin.)
+# But aside from that, the Mingw-w64 nomenclature is rather messy.
+export CI_CC="$CI_TARGET_ARCH-w64-mingw32-gcc"
+export CI_AR="$CI_CC-ar"
+export CI_RANLIB="$CI_CC-ranlib"
+
+export CI_CMAKE_VARS="
+    -DCMAKE_SYSTEM_NAME=Windows
+    -DCMAKE_SYSTEM_PROCESSOR=$CI_TARGET_ARCH
+"