Replace cxx17_bm_searcher config check with feature test macro

This enables the code also for boostrap.

Amends b24e689cb5.

Pick-to: 6.3
Change-Id: I8efc76c20690282a2428f360d6b93896966b97ee
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/configure.cmake

@@ -156,28 +156,6 @@ std::mt19937 mt(0);
 }
 ")
 
-# cxx17_bm_searcher
-qt_config_compile_test(cxx17_bm_searcher
-    LABEL "C++17 boyer_moore_searcher"
-    CODE
-"#include <algorithm>
-#include <functional>
-
-int main(void)
-{
-    /* BEGIN TEST: */
-const char haystack[] = \"hello\";
-const char needle[] = \"e\";
-const auto it =
-std::search(haystack + 0, haystack + std::size(haystack),
-std::boyer_moore_searcher(needle + 0, needle + std::size(needle)));
-(void)it;
-    /* END TEST: */
-    return 0;
-}
-"# FIXME: qmake: CONFIG += c++17
-)
-
 # cxx17_filesystem
 qt_config_compile_test(cxx17_filesystem
     LABEL "C++17 <filesystem>"
@@ -541,10 +519,6 @@ qt_feature("cxx11_future" PUBLIC
     LABEL "C++11 <future>"
     CONDITION TEST_cxx11_future
 )
-qt_feature("cxx17_bm_searcher" PRIVATE
-    LABEL "C++17 boyer_moore_searcher"
-    CONDITION TEST_cxx17_bm_searcher
-)
 qt_feature("cxx17_filesystem" PUBLIC
     LABEL "C++17 <filesystem>"
     CONDITION TEST_cxx17_filesystem

src/corelib/global/qconfig-bootstrapped.h

@@ -78,7 +78,6 @@
 #define QT_FEATURE_cborstreamwriter 1
 #define QT_CRYPTOGRAPHICHASH_ONLY_SHA1
 #define QT_FEATURE_cxx11_random (__has_include(<random>) ? 1 : -1)
-#define QT_FEATURE_cxx17_bm_searcher -1
 #define QT_FEATURE_cxx17_filesystem -1
 #define QT_NO_DATASTREAM
 #define QT_FEATURE_datestring 1

src/corelib/text/qstring.cpp

@@ -10564,7 +10564,7 @@ qsizetype QtPrivate::findString(QLatin1String haystack, qsizetype from, QLatin1S
         return -1;
     }
 
-#if QT_CONFIG(cxx17_bm_searcher)
+#ifdef __cpp_lib_boyer_moore_searcher
     const auto ciHasher = [](char a) { return latin1Lower[uchar(a)]; };
     const auto ciEqual = [](char a, char b) {
         return latin1Lower[uchar(a)] == latin1Lower[uchar(b)];