meson.build: work around Meson bug detecting strndup() on Windows

Work around Meson bug https://github.com/mesonbuild/meson/issues/3672

NEWS

@@ -5,6 +5,7 @@ ver 0.21.24 (not yet released)
   - simple: fix crash when mounting twice
 * fix unit test failure
 * fix build failure with Android NDK r21
+* work around Meson bug breaking the Windows build with GCC 10
 
 ver 0.21.23 (2020/04/23)
 * protocol

meson.build

@@ -142,7 +142,13 @@ conf.set('HAVE_GETPWNAM_R', compiler.has_function('getpwnam_r'))
 conf.set('HAVE_GETPWUID_R', compiler.has_function('getpwuid_r'))
 conf.set('HAVE_INITGROUPS', compiler.has_function('initgroups'))
 conf.set('HAVE_FNMATCH', compiler.has_function('fnmatch'))
-conf.set('HAVE_STRNDUP', compiler.has_function('strndup', prefix: '#define _GNU_SOURCE\n#include <string.h>'))
+
+# Explicitly exclude Windows in this check because
+# https://github.com/mesonbuild/meson/issues/3672 (reported in 2018,
+# still not fixed in 2020) causes Meson to believe it exists, because
+# __builtin_strndup() exists (but strndup() still cannot be used).
+conf.set('HAVE_STRNDUP', not is_windows and compiler.has_function('strndup', prefix: '#define _GNU_SOURCE\n#include <string.h>'))
+
 conf.set('HAVE_STRCASESTR', compiler.has_function('strcasestr'))
 
 conf.set('HAVE_PRCTL', is_linux)