Simplify strcasestr check in fileinfo (#13365)

The strcasestr is not present on Windows and on Solaris 10 until Solaris
11 implementation in string.h.

At this point strcasestr is not used in the bundled libmagic (file)
library due to patch removing the strcasestr usage in file.c, however
future libmagic version bump might need it again. This simplifies the
strcasestr check if available on the system without running the test
code and avoiding the unknown issue when cross-compiling. If found, the
HAVE_STRCASESTR is defined, otherwise the strcasestr.c is added to the
build sources.

Author: petk

ext/fileinfo/config.m4

@@ -14,40 +14,10 @@ if test "$PHP_FILEINFO" != "no"; then
     libmagic/readcdf.c libmagic/softmagic.c libmagic/der.c \
     libmagic/buffer.c libmagic/is_csv.c"
 
-  AC_MSG_CHECKING([for strcasestr])
-  AC_RUN_IFELSE([AC_LANG_SOURCE([[
-#include <string.h>
-#include <strings.h>
-#include <stdlib.h>
-
-int main(void)
-{
-        char *s0, *s1, *ret;
-
-        s0 = (char *) malloc(42);
-        s1 = (char *) malloc(8);
-
-        memset(s0, 'X', 42);
-        s0[24] = 'Y';
-        s0[26] = 'Z';
-        s0[41] = '\0';
-        memset(s1, 'x', 8);
-        s1[0] = 'y';
-        s1[2] = 'Z';
-        s1[7] = '\0';
-
-        ret = strcasestr(s0, s1);
-
-        return !(NULL != ret);
-}
-  ]])],[
-    dnl using the platform implementation
-    AC_MSG_RESULT(yes)
-  ],[
-    AC_MSG_RESULT(no)
+  AC_CHECK_FUNCS([strcasestr],,[
     AC_MSG_NOTICE(using libmagic strcasestr implementation)
     libmagic_sources="$libmagic_sources libmagic/strcasestr.c"
-  ],[AC_MSG_RESULT([skipped, cross-compiling])])
+  ])
 
   PHP_NEW_EXTENSION(fileinfo, fileinfo.c php_libmagic.c $libmagic_sources, $ext_shared,,-I@ext_srcdir@/libmagic)
   PHP_ADD_BUILD_DIR($ext_builddir/libmagic)