Add Meson build project file.

Example usage:

  # Configure Meson build in directory `build-meson` to generate
  # release binaries comparable to to the ones from the
  # autotools/make build system.
  meson setup build-meson \
        --prefix=/usr/local \
        --buildtype=debugoptimized \
        --strip \
        -Db_ndebug=true

  # After configuring the Meson build with the above command,
  # compile and install to `/usr/local/`; this includes a pkg-config
  # file.
  ninja -C build-meson install

  # Alternatively, compile and install to `/tmp/aa/usr/local/...`
  # for packaging.
  DESTDIR=/tmp/aa ninja -C build-meson install

  # Generate documentation under `build-meson/docs`.
  ninja -C build-meson docs

Library size comparison for stripped `libfreetype.so` generated by
all three build systems:

  - Default build (autotools + libtool): 712 KiB
  - CMake build (RelWithDebInfo):        712 KiB
  - Meson build:                         712 KiB

* meson.build: New top-level Meson build file for the library.

* meson_options.txt: New file.  It holds user-selectable options for
the build, which can be printed with `meson configure`, and selected
at `meson setup` or `meson --reconfigure` time with
`-D<option>=<value>`.

* scripts/parse_modules_cfg.py: A script invoked by `meson.build` to
parse `modules.cfg` and extract important information out of it
(i.e., the list of modules).

* scripts/process_ftoption_h.py: New script invoked by `meson.build`
to process the original `ftoption.h` file.  It enables or disables
configuration macro variables based on the available dependencies.
This is similar to what other build systems are using (i.e., Meson's
`configure_file()` command is not used here).

* scripts/extract_freetype_version.py: New script invoked by
`meson.build` to extract the FreeType version number from
`<freetype/freetype.h>`.

* scripts/extract_libtool_version.py: New script invoked by
`meson.build` to extract the libtool `revision_info` data from
`builds/unix/configure.raw`, and to generate the corresponding
shared library suffix.

* scripts/generate_reference_docs.py: New script invoked by
`meson.build` to generate the FreeType 2 reference documentation
(using the `docwriter` and `mkdocs` packages, which must be already
installed).

meson_options.txt

@@ -0,0 +1,47 @@
+#
+# meson_options.txt
+#
+
+# Copyright (C) 2020 by
+# David Turner, Robert Wilhelm, and Werner Lemberg.
+#
+# This file is part of the FreeType project, and may only be used, modified,
+# and distributed under the terms of the FreeType project license,
+# LICENSE.TXT.  By continuing to use, modify, or distribute this file you
+# indicate that you have read the license and understand and accept it
+# fully.
+
+
+option('zlib',
+  type: 'combo',
+  choices: ['disabled', 'auto', 'builtin', 'system'],
+  value: 'auto',
+  description: 'Support reading gzip-compressed font files.')
+
+option('bzip2',
+  type: 'feature',
+  value: 'auto',
+  description: 'Support reading bzip2-compressed font files.')
+
+option('png',
+  type: 'feature',
+  value: 'auto',
+  description: 'Support color bitmap glyph formats in the PNG format.'
+               + 'Requires libpng.')
+
+option('harfbuzz',
+  type: 'feature',
+  value: 'auto',
+  description: 'Use Harfbuzz library to improve auto-hinting.'
+               + ' If available, many glyphs not directly addressable'
+               + ' by a font\'s character map will be hinted also.')
+
+option('brotli',
+  type: 'feature',
+  value: 'auto',
+  description: 'Use Brotli library to support decompressing WOFF2 fonts.')
+
+option('mmap',
+  type: 'feature',
+  value: 'auto',
+  description: 'Use mmap() to open font files for faster parsing.')