* meson.build: Fix 'png' build option (#59458).

Without this patch, 'png' is always required.
diff --git a/ChangeLog b/ChangeLog
index 90c97e5..1354513 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2021-01-06  Torsten Hilbrich  <emacs.nolkaf@hilbrich.tk>
+
+	* meson.build: Fix 'png' build option (#59458).
+
+	Without this patch, 'png' is always required.
+
 2021-01-04  Alexei Podtelezhnikov  <apodtele@gmail.com>
 
 	[builds/windows] Add SDF to VC2010 project.
diff --git a/meson.build b/meson.build
index d0f5783..895ebb3 100644
--- a/meson.build
+++ b/meson.build
@@ -264,8 +264,10 @@
 
 # PNG support
 libpng_dep = dependency('libpng', required: get_option('png'))
-ftoption_command += ['--enable=FT_CONFIG_OPTION_USE_PNG']
-ft2_deps += [libpng_dep]
+if libpng_dep.found()
+  ftoption_command += ['--enable=FT_CONFIG_OPTION_USE_PNG']
+  ft2_deps += [libpng_dep]
+endif
 
 # Harfbuzz support
 harfbuzz_dep = dependency('harfbuzz',