Consistently use SAL_USE_VCLPLUGIN as a bootstrap variable

...and neither exclusively nor additionally as an environment variable (where
bootstrap variables include environment variables, see the @file comment in
include/rtl/bootstrap.h).  Note that LibLibreOffice_Impl::initialize
(desktop/source/lib/init.cxx) sets it as a bootstrap, not as an environment
variable.  (Though desktop/unx/source/start.c still only reads it as an
environment variable; could potentially check argv for
-env:SAL_USE_VCLPLUGIN=...)

Change-Id: I9315f585cdb4f0783181359e030686167c4e4625

vcl/unx/generic/desktopdetect/desktopdetector.cxx

@@ -24,6 +24,7 @@
 
 #include <unx/desktops.hxx>
 
+#include "rtl/bootstrap.hxx"
 #include "rtl/process.h"
 #include "rtl/ustrbuf.hxx"
 #include "osl/module.h"
@@ -303,10 +304,10 @@ DESKTOP_DETECTOR_PUBLIC DesktopType get_desktop_environment()
     // get display to connect to
     const char* pDisplayStr = getenv( "DISPLAY" );
 
-    const char* pUsePlugin = getenv( "SAL_USE_VCLPLUGIN" );
+    OUString plugin;
+    rtl::Bootstrap::get("SAL_USE_VCLPLUGIN", plugin);
 
-    if ((pUsePlugin && (strcmp(pUsePlugin, "svp") == 0))
-        || Application::IsHeadlessModeRequested())
+    if (plugin == "svp" || Application::IsHeadlessModeRequested())
         pDisplayStr = NULL;
     else
     {

vcl/unx/generic/plugadapt/salplug.cxx

@@ -220,11 +220,7 @@ SalInstance *CreateSalInstance()
         aUsePlugin = "svp";
     else
     {
-        static const char* pUsePlugin = getenv( "SAL_USE_VCLPLUGIN" );
-        if( pUsePlugin )
-            aUsePlugin = OUString::createFromAscii( pUsePlugin );
-        else
-            rtl::Bootstrap::get( "SAL_USE_VCLPLUGIN", aUsePlugin );
+        rtl::Bootstrap::get( "SAL_USE_VCLPLUGIN", aUsePlugin );
     }
 
     if( !aUsePlugin.isEmpty() )