Add --with-exr toggle so USE=-openexr actually disables detection.

Upstream auto-enables OpenEXR whenever a C++ compiler is available,
which turns USE=-openexr into a no-op on Gentoo. Wrap the probe in a
GWY_WITH option defaulting to $have_cxx, matching the upstream pattern
for cfitsio/webp/json-glib.

--- a/configure.ac
+++ b/configure.ac
@@ -581,7 +581,7 @@
 # OpenEXR 2.x+
 # Optional.  Used for HDR greyscale OpenEXR pixmap import/export. Only try to
 # enable if we have a C++ compiler because it is a C++ library.
-enable_exr=$have_cxx
+GWY_WITH([exr],,[build with OpenEXR support],[$have_cxx])
 if test "x$enable_exr" != xno; then
   PKG_CHECK_MODULES(EXR, [OpenEXR >= 2.0],
                     [AC_DEFINE(HAVE_EXR,1,