libtiff: hide implementation details

building with -DLOGLUV_PUBLIC=0 will convert some libtiff functions to
"static", preventing potential name clashes with downstream.

Pick-to: 6.10
Task-number: QTBUG-138543
Change-Id: I3808de27121ac975714b17508b20cb4bc5203e23
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

src/3rdparty/libtiff/libtiff/tiffio.h

@@ -590,7 +590,7 @@ extern int TIFFReadRGBAImageOriented(TIFF *, uint32_t, uint32_t, uint32_t *,
     extern uint64_t TIFFGetStrileByteCountWithErr(TIFF *tif, uint32_t strile,
                                                   int *pbErr);
 
-#ifdef LOGLUV_PUBLIC
+#if LOGLUV_PUBLIC
 #define U_NEU 0.210526316
 #define V_NEU 0.473684211
 #define UVSCALE 410.

src/3rdparty/libtiff/patches/0001-tiffio.h-fix-compilation-with-LOGLUV_PUBLIC-0.patch

@@ -0,0 +1,32 @@
+From c7ea716f16a0eb018c791d05623ce7ae050c8064 Mon Sep 17 00:00:00 2001
+From: Tim Blechmann <tim@klingt.org>
+Date: Tue, 12 Aug 2025 16:59:50 +0800
+Subject: [PATCH] tiffio.h: fix compilation with LOGLUV_PUBLIC=0
+
+LOGLUV_PUBLIC can be used to compile some functions with hidden
+visibility.
+However this functionality was broken (in clang), as LOGLUV_PUBLIC was
+defined to 1 if it wasn't defined, as the extern declarations were still
+visible by the compiler leading to "static declaration follows
+non-static declaration" errors.
+Changing the header from #ifdef to #if fixes the issue.
+---
+ libtiff/tiffio.h | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/libtiff/tiffio.h b/libtiff/tiffio.h
+index 6a7ab2f5..f9c206e3 100644
+--- a/libtiff/tiffio.h
++++ b/libtiff/tiffio.h
+@@ -608,7 +608,7 @@ extern int TIFFReadRGBAImageOriented(TIFF *, uint32_t, uint32_t, uint32_t *,
+     extern uint64_t TIFFGetStrileByteCountWithErr(TIFF *tif, uint32_t strile,
+                                                   int *pbErr);
+ 
+-#ifdef LOGLUV_PUBLIC
++#if LOGLUV_PUBLIC
+ #define U_NEU 0.210526316
+ #define V_NEU 0.473684211
+ #define UVSCALE 410.
+-- 
+2.50.1
+

src/plugins/imageformats/tiff/CMakeLists.txt

@@ -86,6 +86,8 @@ qt_internal_extend_target(QTiffPlugin CONDITION NOT QT_FEATURE_system_tiff
         ../../../3rdparty/libtiff/libtiff
     ATTRIBUTION_FILE_DIR_PATHS
         ../../../3rdparty/libtiff
+    DEFINES
+        LOGLUV_PUBLIC=0
 )
 
 qt_internal_extend_target(QTiffPlugin CONDITION WIN32 AND NOT QT_FEATURE_system_tiff