Make WINDOW_G configurable

This makes WINDOW_G a configurable value in the range of [2..24].
The upper limit of 24 is a defensive choice. The code is probably
correct for values up to 27 but those larger values yield in huge
tables (>= 256MiB), which are i) unlikely to be really beneficial
in practice and ii) increasingly difficult to test.

src/basic-config.h

@@ -28,6 +28,7 @@
 #define USE_SCALAR_INV_BUILTIN 1
 #define USE_FIELD_10X26 1
 #define USE_SCALAR_8X32 1
+#define ECMULT_WINDOW_SIZE 15
 
 #endif /* USE_BASIC_CONFIG */