Probe for strcasestr using the C++ compiler

The C++ compiler may define _GNU_SOURCE, causing <string.h> to
declare strcasestr, while the C compiler does not.  As a result,
the probe may fail, resulting in an incompatible declaration
of strcasestr in compat/compat.h.

Upstream discussion:

  [PATCH] compat/strcasestr: Include correct header file
  <https://nmbug.notmuchmail.org/nmweb/show/20221202191908.848227-1-qsx%40chaotikum.eu>

diff --git a/compat/have_strcasestr.c b/compat/have_strcasestr.c
index 3cd1838d950bd30c..af0a2db7046c1170 100644
--- a/compat/have_strcasestr.c
+++ b/compat/have_strcasestr.c
@@ -1,4 +1,5 @@
 #define _GNU_SOURCE
+#include <string.h>
 #include <strings.h>
 
 int