[meson] Fix 'mmap' handling on Windows.

* meson.build (ft2_sources): Always use `windows/ftsystem.c` on
Windows unless mmap has been disabled.

  https://gitlab.freedesktop.org/freetype/freetype/-/merge_requests/5#note_829289

ChangeLog

@@ -1,3 +1,12 @@
+2021-03-08  Tim-Philipp Müller  <tim@centricular.com>
+
+	[meson] Fix 'mmap' handling on Windows.
+
+	* meson.build (ft2_sources): Always use `windows/ftsystem.c` on
+	Windows unless mmap has been disabled.
+
+	  https://gitlab.freedesktop.org/freetype/freetype/-/merge_requests/5#note_829289
+
 2021-03-07  Tim-Philipp Müller  <tim@centricular.com>
 
 	[ci] Add basic support for Meson builds with MSVC on Windows.

meson.build

@@ -183,18 +183,20 @@ has_fcntl_h = cc.has_header('fcntl.h')
 has_sys_mman_h = cc.has_header('sys/mman.h')
 
 mmap_option = get_option('mmap')
-if mmap_option.auto()
-  use_mmap = has_unistd_h and has_fcntl_h and has_sys_mman_h
-else
-  use_mmap = mmap_option.enabled()
-endif
-if use_mmap
-  # This version of ftsystem.c uses mmap() to read input font files.
-  ft2_sources += files(['builds/unix/ftsystem.c',])
+if mmap_option.disabled()
+  ft2_sources += files(['src/base/ftsystem.c',])
 elif host_machine.system() == 'windows'
   ft2_sources += files(['builds/windows/ftsystem.c',])
 else
-  ft2_sources += files(['src/base/ftsystem.c',])
+  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',])
+  elif mmap_option.enabled()
+    error('mmap was enabled via options but is not available,'
+          + ' required headers were not found!')
+  else
+    ft2_sources += files(['src/base/ftsystem.c',])
+  endif
 endif