Meson: Fix dependency lookup and generate ftconfig.h

[meson] Fix dependency lookup and generate `ftconfig.h`.

- zlib: If not found on the system, meson can build it as a
  subproject.  We thus never use the (outdated) zlib support that
  comes with FreeType.  Doing so has the additional advantage that
  the zlib code can be shared with other projects like GLib if both
  are subprojects of an application.
- harfbuzz: Build as a subproject if not found on the system.
- 'QUESTION: What if the compiler doesn't support `-D` but uses `/D`
  instead as on Windows?'  Answer: Meson translate arguments for us.
- visibility: Replace self-made code with meson-specific solution.

* meson.build (ft2_defines): Rewrite logic to set and handle it.
(process_header_command): New variable, previously called
`ftoption_command`.
(ftoption_command, ftconfig_command): New variables.
(zlib_option): Removed.
(zlib_dep): New variable.
(ft2_deps): Updated.
(harfbuzz_dep): Updated.
(ftconfig_h_in, ftconfig_h): New variables.
(ft2_sources): Updated.
(ft2_lib): Updated, handle visibility.
(summary): Updted.

* meson_options.txt (zlib): Updated.

1 files changed, 1 insertions, 2 deletions

diff --git a/meson_options.txt b/meson_options.txt
index 301acdccb..f0fc50a38 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -13,8 +13,7 @@
 
 
 option('zlib',
-  type: 'combo',
-  choices: ['disabled', 'auto', 'builtin', 'system'],
+  type: 'feature',
   value: 'auto',
   description: 'Support reading gzip-compressed font files.')