MSVC: compat: strcasecmp() in <strings.h>

The #ifdef is for `_MSC_VER`, not `_WIN32`, because it's apparently
available on MinGW.
diff --git a/compat/compat.h.in b/compat/compat.h.in
index 1c19b98..7c641f7 100644
--- a/compat/compat.h.in
+++ b/compat/compat.h.in
@@ -66,6 +66,7 @@
 #cmakedefine HAVE_STRPTIME
 #cmakedefine HAVE_MMAP
 #cmakedefine HAVE_DIRNAME
+#cmakedefine HAVE_STRCASECMP
 
 #ifndef bswap64
 #define bswap64(val) \