configure: Add configure-time libmagick indication.

configure.ac

@@ -1892,6 +1892,14 @@ LDFLAGS=$restore_ldflags
 
 AM_CONDITIONAL([IMAGEMAGICK], [test "x$have_imagemagick" = "xyes"])
 
+dnl IRCD_ENABLE_MAGICK is always defined for use in a constexpr if
+AM_COND_IF([IMAGEMAGICK],
+[
+	IRCD_DEFINE(USE_MAGICK, [1], [Magick support is available and enabled])
+], [
+	IRCD_DEFINE(USE_MAGICK, [0], [Magick support is unavailable or disabled])
+])
+
 dnl
 dnl
 dnl boost support

include/ircd/magick.h

@@ -110,3 +110,17 @@ struct ircd::magick::job
 	string_view description;   // only valid for current job duration
 	std::exception_ptr eptr;   // apropos exception reference
 };
+
+// inline stubs when magick disabled/unavailable.
+#if !IRCD_USE_MAGICK
+
+inline
+ircd::magick::init::init()
+{}
+
+inline
+ircd::magick::init::~init()
+noexcept
+{}
+
+#endif IRCD_USE_MAGICK

modules/media/thumbnail.cc

@@ -292,8 +292,11 @@ get__thumbnail_local(client &client,
 
 	const bool fallback // Reasons to just send the original image
 	{
+		// Not available in build
+		!IRCD_USE_MAGICK
+
 		// Disabled by configuration
-		!enable
+		|| !enable
 
 		// Access denied for this operation
 		|| !permitted