diff -urN pdfium/core/fxcrt/fx_extension.cpp pdfium.new/core/fxcrt/fx_extension.cpp
--- core/fxcrt/fx_extension.cpp	2025-02-10 19:49:22.000000000 +0100
+++ core/fxcrt/fx_extension.cpp	2025-09-21 11:53:43.310869830 +0200
@@ -16,7 +16,11 @@
 #include "core/fxcrt/fx_system.h"
 #include "core/fxcrt/utf16.h"
 #include "core/fxcrt/widestring.h"
+#ifdef USE_SYSTEM_FAST_FLOAT
+#include <fast_float/fast_float.h>
+#else
 #include "third_party/fast_float/src/include/fast_float/fast_float.h"
+#endif
 
 namespace {
 
diff -urN pdfium/core/fxcrt/fx_string.cpp pdfium.new/core/fxcrt/fx_string.cpp
--- core/fxcrt/fx_string.cpp	2025-02-10 19:49:22.000000000 +0100
+++ core/fxcrt/fx_string.cpp	2025-09-21 11:53:03.382500482 +0200
@@ -20,7 +20,11 @@
 #include "core/fxcrt/span.h"
 #include "core/fxcrt/utf16.h"
 #include "core/fxcrt/widestring.h"
+#ifdef USE_SYSTEM_FAST_FLOAT
+#include <fast_float/fast_float.h>
+#else
 #include "third_party/fast_float/src/include/fast_float/fast_float.h"
+#endif
 
 #if !defined(WCHAR_T_IS_16_BIT) && !defined(WCHAR_T_IS_32_BIT)
 #error "Unknown wchar_t size"