fix sysrootification of install paths

initially, the idea was that QLibraryInfo would receive a
pre-sysrootified ExtPrefix from the builtin qt.conf. matching this
against the sysroot would then tell us whether to sysrootify the Prefix
from a "regular" qt.conf as well.
however, this would have lead to some major ugliness and inconsistency
between the code paths, so i changed my mind.
unfortunately, i failed to adjust the remaining code, leading to
169a40d51 entirely breaking sysrootification ...

the proper (and nicely consistent) solution is to introduce a
SysrootifyPrefix key to qt.conf. this is user-accessible as well, so as
a bonus it is now possible to adjust the setting at qmake installation
time. incidentally, this omission was the last thing that prevented
using the same qmake host build for any imaginable configuration of the
same qt version ... i think.

Change-Id: Ic0eebf21f93651f6374628c0ad8b206d696a4a7e
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

configure.pri

@@ -680,6 +680,7 @@ defineReplace(printHostPaths) {
         $$printInstallPath(HostLibraries, hostlibdir, lib) \
         $$printInstallPath(HostData, hostdatadir, .) \
         "Sysroot=$$config.input.sysroot" \
+        "SysrootifyPrefix=$$qmake_sysrootify" \
         "TargetSpec=$$XSPEC" \
         "HostSpec=$$[QMAKE_SPEC]"
     return($$ret)

src/corelib/global/qlibraryinfo.cpp

@@ -113,13 +113,6 @@ public:
                          ? ls->haveDevicePaths
                          : ls->havePaths) : false;
     }
-    static bool sysrootify()
-    {
-        // This is actually bogus, as it does not consider post-configure settings.
-        QLibrarySettings *ls = qt_library_settings();
-        return ls ? (!ls->builtinValues[QLibraryInfo::SysrootPath].isEmpty()
-                     && ls->builtinValues[QLibraryInfo::ExtPrefixPath].isEmpty()) : false;
-    }
     static QString builtinValue(int loc)
     {
         QLibrarySettings *ls = qt_library_settings();
@@ -483,6 +476,7 @@ static const struct {
     { "Tests", "tests" },
 #ifdef QT_BUILD_QMAKE
     { "Sysroot", "" },
+    { "SysrootifyPrefix", "" },
     { "HostBinaries", "bin" },
     { "HostLibraries", "lib" },
     { "HostData", "." },
@@ -522,13 +516,17 @@ QLibraryInfo::location(LibraryLocation loc)
     QString ret = rawLocation(loc, FinalPaths);
 
     // Automatically prepend the sysroot to target paths
-    if ((loc < SysrootPath || loc > LastHostPath) && QLibraryInfoPrivate::sysrootify()) {
+    if (loc < SysrootPath || loc > LastHostPath) {
         QString sysroot = rawLocation(SysrootPath, FinalPaths);
-        if (!sysroot.isEmpty() && ret.length() > 2 && ret.at(1) == QLatin1Char(':')
-            && (ret.at(2) == QLatin1Char('/') || ret.at(2) == QLatin1Char('\\')))
-            ret.replace(0, 2, sysroot); // Strip out the drive on Windows targets
-        else
-            ret.prepend(sysroot);
+        if (!sysroot.isEmpty()
+                && QVariant::fromValue(rawLocation(SysrootifyPrefixPath, FinalPaths)).toBool()) {
+            if (ret.length() > 2 && ret.at(1) == QLatin1Char(':')
+                   && (ret.at(2) == QLatin1Char('/') || ret.at(2) == QLatin1Char('\\'))) {
+                ret.replace(0, 2, sysroot); // Strip out the drive on Windows targets
+            } else {
+                ret.prepend(sysroot);
+            }
+        }
     }
 
     return ret;
@@ -591,7 +589,7 @@ QLibraryInfo::rawLocation(LibraryLocation loc, PathGroup group)
                 if (loc == HostPrefixPath)
                     ret = config->value(QLatin1String(qtConfEntries[PrefixPath].key),
                                         QLatin1String(qtConfEntries[PrefixPath].value)).toString();
-                else if (loc == TargetSpecPath || loc == HostSpecPath)
+                else if (loc == TargetSpecPath || loc == HostSpecPath || loc == SysrootifyPrefixPath)
                     fromConf = false;
                 // The last case here is SysrootPath, which can be legitimately empty.
                 // All other keys have non-empty fallbacks to start with.
@@ -645,8 +643,8 @@ QLibraryInfo::rawLocation(LibraryLocation loc, PathGroup group)
     }
 
 #ifdef QT_BUILD_QMAKE
-    // The specs need to be returned verbatim.
-    if (loc == TargetSpecPath || loc == HostSpecPath)
+    // These values aren't actually paths and thus need to be returned verbatim.
+    if (loc == TargetSpecPath || loc == HostSpecPath || loc == SysrootifyPrefixPath)
         return ret;
 #endif
 

src/corelib/global/qlibraryinfo.h

@@ -91,6 +91,7 @@ public:
 #ifdef QT_BUILD_QMAKE
         // These are not subject to binary compatibility constraints
         SysrootPath,
+        SysrootifyPrefixPath,
         HostBinariesPath,
         HostLibrariesPath,
         HostDataPath,