SAL_USE_VCLPLUGIN is a UNO bootstrap variable

...(which includes environment variables), so read it as one (as is already done
in e.g. CreateSalInstance in vcl/source/app/salplug.cxx), esp. so since
lo_initialize in desktop/source/lib/init.cxx doesn't set it as an environment
variable

Change-Id: I557ecf4fcc5daba1e748af789847c8bbb9f563d6
Reviewed-on: https://gerrit.libreoffice.org/63801Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

vcl/source/opengl/OpenGLHelper.cxx

@@ -987,8 +987,11 @@ bool OpenGLHelper::isVCLOpenGLEnabled()
     else if (bSupportsVCLOpenGL)
     {
         static bool bEnableGLEnv = !!getenv("SAL_ENABLEGL");
-        static bool bHeadlessPlugin = (getenv("SAL_USE_VCLPLUGIN") &&
-            0 == strcmp(getenv("SAL_USE_VCLPLUGIN"), "svp"));
+        static bool bHeadlessPlugin = []{
+            OUString plugin;
+            rtl::Bootstrap::get("SAL_USE_VCLPLUGIN", plugin);
+            return plugin == "svp";
+        }();
 
         bEnable = bEnableGLEnv;