disable PEM support on MSVC

If someone wants to fix builds on MSVC, please step forward. Until then
the library can still be used on Windows via `mingw-gcc`.

Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>

Author: sjaeckel

src/headers/tomcrypt_custom.h

@@ -759,4 +759,11 @@
 #undef LTC_ECC521
 #endif
 
+/* MSVC can't build PEM */
+#if defined(LTC_PEM) && defined(_MSC_VER)
+#undef LTC_PEM
+#undef LTC_PEM_DECODE_BUFSZ
+#undef LTC_PEM_READ_BUFSIZE
+#endif
+
 #endif /* TOMCRYPT_CUSTOM_H_ */