move the setup of QT_INSTALL_DOCS from qdoc to qt_docs.prf

this is a qt specific option and really should not be hard-coded.

also, the implementation used undocumented api that is internal to the
bootstrapped process, which made it impossible to de-bootstrap it.

Change-Id: If706960671744e64a9a7c366437977a800a6058e
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>

mkspecs/features/qt_docs.prf

@@ -32,7 +32,9 @@ qtmver.name = QT_VER
 qtmver.value = $$replace(qtver.value, ^(\\d+\\.\\d+).*$, \\1)
 qtvertag.name = QT_VERSION_TAG
 qtvertag.value = $$replace(qtver.value, \.,)
-qtAddToolEnv(QDOC, qtver qtmver qtvertag)
+qtdocs.name = QT_INSTALL_DOCS
+qtdocs.value = $$[QT_INSTALL_DOCS/get]
+qtAddToolEnv(QDOC, qtver qtmver qtvertag qtdocs)
 doc_command = $$QDOC $$QMAKE_DOCS
 prepare_docs {
     prepare_docs.commands += $$doc_command -prepare -no-link-errors

src/tools/qdoc/main.cpp

@@ -40,7 +40,6 @@
 ****************************************************************************/
 
 #include <qglobal.h>
-#include <qlibraryinfo.h>
 #include <stdlib.h>
 #include "codemarker.h"
 #include "codeparser.h"
@@ -261,17 +260,6 @@ static void processQdocconfFile(const QString &fileName)
     config.setStringList(CONFIG_NOLINKERRORS, QStringList(noLinkErrors ? "true" : "false"));
     config.setStringList(CONFIG_OBSOLETELINKS, QStringList(obsoleteLinks ? "true" : "false"));
 
-    /*
-      If QT_INSTALL_DOCS is not set, set it here so it can be used from
-      the qdocconf files.
-    */
-    QString qt_install_docs = qgetenv("QT_INSTALL_DOCS");
-    if (qt_install_docs.isEmpty()) {
-        documentationPath = QLibraryInfo::rawLocation(QLibraryInfo::DocumentationPath,
-                                                      QLibraryInfo::EffectivePaths);
-        qputenv("QT_INSTALL_DOCS", documentationPath.toLatin1());
-    }
-
     /*
       With the default configuration values in place, load
       the qdoc configuration file. Note that the configuration