* meson.build: Fix build for other UNIX systems (e.g., FreeBSD).

Without this change the build of `unix/ftsystem.c` fails because the
`ftconfig.h` header that defines macros such as `HAVE_UNISTD_H` and
`HAVE_FCNTL_H` is only being generated for Linux, macOS, and Cygwin
systems:

```
.../builds/unix/ftsystem.c:258:32: error:
    use of undeclared identifier 'O_RDONLY'
file = open( filepathname, O_RDONLY );
```

Instead of hardcoding a list of operating systems for this check,
update the logic that decides whether to build the file and set a
boolean flag that can be checked instead.

ChangeLog

@@ -1,3 +1,22 @@
+2021-07-12  Alex Richardson  <Alexander.Richardson@cl.cam.ac.uk>
+
+	* meson.build: Fix build for other UNIX systems (e.g., FreeBSD).
+
+	Without this change the build of `unix/ftsystem.c` fails because the
+	`ftconfig.h` header that defines macros such as `HAVE_UNISTD_H` and
+	`HAVE_FCNTL_H` is only being generated for Linux, macOS, and Cygwin
+	systems:
+
+	```
+	.../builds/unix/ftsystem.c:258:32: error:
+	    use of undeclared identifier 'O_RDONLY'
+	file = open( filepathname, O_RDONLY );
+	```
+
+	Instead of hardcoding a list of operating systems for this check,
+	update the logic that decides whether to build the file and set a
+	boolean flag that can be checked instead.
+
 2021-07-12  Werner Lemberg  <wl@gnu.org>
 
 	[autofit] More clean-ups.

meson.build

@@ -193,6 +193,7 @@ has_sys_mman_h = cc.has_header('sys/mman.h')
 
 mmap_option = get_option('mmap')
 
+use_unix_ftsystem_c = false
 if mmap_option.disabled()
   ft2_sources += files(['src/base/ftsystem.c',])
 elif host_machine.system() == 'windows'
@@ -201,6 +202,7 @@ else
   if has_unistd_h and has_fcntl_h and has_sys_mman_h
     # This version of `ftsystem.c` uses `mmap` to read input font files.
     ft2_sources += files(['builds/unix/ftsystem.c',])
+    use_unix_ftsystem_c = true
   elif mmap_option.enabled()
     error('mmap was enabled via options but is not available,'
           + ' required headers were not found!')
@@ -321,7 +323,7 @@ if has_fcntl_h
   ftconfig_command += '--enable=HAVE_FCNTL_H'
 endif
 
-if host_machine.system() in ['linux', 'darwin', 'cygwin']
+if use_unix_ftsystem_c
   ftconfig_h_in = files('builds/unix/ftconfig.h.in')
   ftconfig_h = custom_target('ftconfig.h',
     input: ftconfig_h_in,