add --vips-config flag

so "vips --vips-config" now displays:

```
$ vips --vips-config
native win32: no, native OS X: no, open files in binary mode: no, ...
```

ChangeLog

@@ -28,6 +28,7 @@
   vips_autorot_get_angle() [Elad-Laufer]
 - deprecate heifload autorotate -- it's now always on
 - revised resize improves accuracy [kleisauke]
+- add --vips-config flag to show configuration info
 
 24/4/20 started 8.9.3
 - better iiif tile naming [IllyaMoskvin]

configure.ac

@@ -1307,11 +1307,15 @@ VIPS_CFLAGS="$VIPS_DEBUG_FLAGS $VIPS_CFLAGS"
 VIPS_INCLUDES="$ZLIB_INCLUDES $PNG_INCLUDES $TIFF_INCLUDES $JPEG_INCLUDES $NIFTI_INCLUDES" 
 VIPS_LIBS="$ZLIB_LIBS $HEIF_LIBS $MAGICK_LIBS $PNG_LIBS $IMAGEQUANT_LIBS $TIFF_LIBS $JPEG_LIBS $GTHREAD_LIBS $GIO_LIBS $REQUIRED_LIBS $EXPAT_LIBS $PANGOFT2_LIBS $GSF_LIBS $FFTW_LIBS $ORC_LIBS $LCMS_LIBS $GIFLIB_LIBS $RSVG_LIBS $NIFTI_LIBS $PDFIUM_LIBS $POPPLER_LIBS $OPENEXR_LIBS $OPENSLIDE_LIBS $CFITSIO_LIBS $LIBWEBP_LIBS $LIBWEBPMUX_LIBS $MATIO_LIBS $EXIF_LIBS -lm"
 
+# autoconf hates multi-line AC_SUBST
+VIPS_CONFIG="native win32: $vips_os_win32, native OS X: $vips_os_darwin, open files in binary mode: $vips_binary_open, enable debug: $enable_debug, enable deprecated library components: $enable_deprecated, enable docs with gtkdoc: $enable_gtk_doc, gobject introspection: $found_introspection, enable radiance support: $with_radiance, enable analyze support: $with_analyze, enable PPM support: $with_ppm, use fftw3 for FFT: $with_fftw, Magick package: $with_magickpackage, Magick API version: $magick_version, load with libMagick: $enable_magickload, save with libMagick: $enable_magicksave, accelerate loops with orc: $with_orc, ICC profile support with lcms: $with_lcms, file import with niftiio: $with_nifti, file import with libheif: $with_heif, file import with OpenEXR: $with_OpenEXR, file import with OpenSlide: $with_openslide, file import with matio: $with_matio, PDF import with PDFium: $with_pdfium, PDF import with poppler-glib: $with_poppler, SVG import with librsvg-2.0: $with_rsvg, zlib: $with_zlib, file import with cfitsio: $with_cfitsio, file import/export with libwebp: $with_libwebp, text rendering with pangoft2: $with_pangoft2, file import/export with libpng: $with_png, support 8bpp PNG quantisation: $with_imagequant, file import/export with libtiff: $with_tiff, file import/export with giflib: $with_giflib, file import/export with libjpeg: $with_jpeg, image pyramid export: $with_gsf, use libexif to load/save JPEG metadata: $with_libexif"
+
 AC_SUBST(VIPS_LIBDIR)
 
 AC_SUBST(VIPS_CFLAGS)
 AC_SUBST(VIPS_INCLUDES)
 AC_SUBST(VIPS_LIBS)
+AC_SUBST(VIPS_CONFIG)
 AC_SUBST(PACKAGES_USED)
 AC_SUBST(EXTRA_LIBS_USED)
 
@@ -1391,7 +1395,7 @@ file import with OpenEXR: 		$with_OpenEXR
 file import with OpenSlide:		$with_openslide
   (requires openslide-3.3.0 or later)
 file import with matio: 		$with_matio
-PDF import with PDFium 			$with_pdfium
+PDF import with PDFium: 		$with_pdfium
 PDF import with poppler-glib: 		$with_poppler
   (requires poppler-glib 0.16.0 or later)
 SVG import with librsvg-2.0: 		$with_rsvg

libvips/include/vips/version.h.in

@@ -16,6 +16,8 @@
 #define VIPS_LIBRARY_REVISION	(@LIBRARY_REVISION@)
 #define VIPS_LIBRARY_AGE	(@LIBRARY_AGE@)
 
+#define VIPS_CONFIG		"@VIPS_CONFIG@"
+
 /** 
  * VIPS_SONAME:
  *

libvips/iofuncs/init.c

@@ -741,6 +741,15 @@ vips_lib_version_cb( const gchar *option_name, const gchar *value,
 	exit( 0 );
 }
 
+static gboolean
+vips_lib_config_cb( const gchar *option_name, const gchar *value, 
+	gpointer data, GError **error )
+{
+	printf( "%s\n", VIPS_CONFIG );
+	vips_shutdown();
+	exit( 0 );
+}
+
 static gboolean
 vips_cache_max_cb( const gchar *option_name, const gchar *value, 
 	gpointer data, GError **error )
@@ -823,6 +832,9 @@ static GOptionEntry option_entries[] = {
 	{ "vips-version", 0, G_OPTION_FLAG_NO_ARG, 
 		G_OPTION_ARG_CALLBACK, (gpointer) &vips_lib_version_cb, 
 		N_( "print libvips version" ), NULL },
+	{ "vips-config", 0, G_OPTION_FLAG_NO_ARG, 
+		G_OPTION_ARG_CALLBACK, (gpointer) &vips_lib_config_cb, 
+		N_( "print libvips config" ), NULL },
 	{ "vips-pipe-read-limit", 0, 0, 
 		G_OPTION_ARG_INT64, (gpointer) &vips_pipe_read_limit, 
 		N_( "read at most this many bytes from a pipe" ), NULL },