Fix the way we get high-resolution time.

If we're not on Windows, use clock_gettime(CLOCK_REALTIME) *if* we have
it; otherwise, fall back on gettimeofday().

(Note: neither Linux, nor macOS, nor Windows necessarily "have"
particular APIs; particular *versions* of Linux distributions
(kernel+libc) have them, particular *versions* of macOS have them, and
particular *versions* of Windows+MSVC have them.

And Linux, Windows and macOS aren't the only platforms on which we run.)

Fixes #17101.

ConfigureChecks.cmake

@@ -94,6 +94,19 @@ set(CMAKE_REQUIRED_LIBRARIES ${M_LIBRARIES})
 check_symbol_exists("floorl" "math.h"    HAVE_FLOORL)
 cmake_pop_check_state()
 
+#
+# Check whether we have clock_gettime().
+# It's not on Windows, so don't waste time checking for it.
+# It's in newer POSIX, so some, but not all, UN*Xes have it.
+#
+if (NOT WIN32)
+	#
+	# POSIX - don't bother checking on Windows, as checks
+	# take time.
+	#
+	check_function_exists("clock_gettime"    HAVE_CLOCK_GETTIME)
+endif (NOT WIN32)
+
 check_function_exists("getopt_long"      HAVE_GETOPT_LONG)
 if(HAVE_GETOPT_LONG)
 	#

cmakeconfig.h.in

@@ -40,6 +40,9 @@
 /* Define to 1 if you have the <arpa/inet.h> header file. */
 #cmakedefine HAVE_ARPA_INET_H 1
 
+/* Define to 1 if you have the `clock_gettime` function. */
+#cmakedefine HAVE_CLOCK_GETTIME 1
+
 /* Define to 1 if you have the <fcntl.h> header file. */
 #cmakedefine HAVE_FCNTL_H 1
 

ui/qt/import_text_dialog.cpp

@@ -10,6 +10,10 @@
 #include "config.h"
 
 #include <time.h>
+#if !defined(_WIN32) && !defined(HAVE_CLOCK_GETTIME)
+// For gettimeofday()
+#include <sys/time.h>
+#endif
 
 #include "import_text_dialog.h"
 
@@ -382,10 +386,21 @@ void ImportTextDialog::on_dateTimeLineEdit_textChanged(const QString &time_forma
             char time_str[100];
             QString timefmt = QString(time_format);
 
-#ifdef _WIN32
-            timespec_get(&timenow, TIME_UTC); /* supported by Linux and Windows */
+#if defined(_WIN32)
+            // At least some Windows C libraries have this.
+            // Some UN*X C libraries do, as well, but they might not
+            // show it unless you're requesting C11 - or C++17.
+            timespec_get(&timenow, TIME_UTC);
+#elif defined(HAVE_CLOCK_GETTIME)
+            // Newer POSIX API.  Some UN*Xes whose C libraries lack
+            // timespec_get() (C11) have this.
+            clock_gettime(CLOCK_REALTIME, &timenow);
 #else
-            clock_gettime(CLOCK_REALTIME, &timenow); /* supported by Linux and MacOS */
+            // Fall back on gettimeofday().
+            struct timeval usectimenow;
+            gettimeofday(&usectimenow, NULL);
+            timenow.tv_sec = usectimenow.tv_sec;
+            timenow.tv_nsec = usectimenow.tv_usec*1000;
 #endif
             /* On windows strftime/wcsftime does not support %s yet, this works on all OSs */
             timefmt.replace(QString("%s"), QString::number(timenow.tv_sec));