#87: update word size detection

The detection for word size was dependent on `__LONG_WIDTH__` for at
least gcc Alpine Linux (probably other musl distros too).

This is not working with clang which does not define `__LONG_WIDTH__`.
Replace `__LONG_WIDTH__` by `__SIZE_WIDTH__`.

As this will not work as expected for x32 ABI, add robust x32 detection
first.

Closes #87.

lib/env.h

@@ -43,12 +43,15 @@
 #endif
 
 // Detect word size:
-#if defined(_INTEGRAL_MAX_BITS)
+#if defined(__x86_64__)
+// This also works for the x32 ABI, which has a 64-bit word size.
+#  define BASE64_WORDSIZE 64
+#elif defined(_INTEGRAL_MAX_BITS)
 #  define BASE64_WORDSIZE _INTEGRAL_MAX_BITS
 #elif defined(__WORDSIZE)
 #  define BASE64_WORDSIZE __WORDSIZE
-#elif defined (__LONG_WIDTH__)
-#  define BASE64_WORDSIZE __LONG_WIDTH__
+#elif defined (__SIZE_WIDTH__)
+#  define BASE64_WORDSIZE __SIZE_WIDTH__
 #else
 #  error BASE64_WORDSIZE_NOT_DEFINED
 #endif