autotools: Remove "auto" value of --with-ecmult-window option

"auto" implies that a value is being chosen based on build system
introspection or host system capabilities. However, for the
`--with-ecmult-window` option, the value "auto" is hardcoded, which
might lead to confusion.

This change replaces "auto" with a more appropriate default value.

.cirrus.yml

@@ -10,7 +10,7 @@ env:
   MAKEFLAGS: -j4
   BUILD: check
   ### secp256k1 config
-  ECMULTWINDOW: auto
+  ECMULTWINDOW: 15
   ECMULTGENKB: auto
   ASM: no
   WIDEMUL: auto

.github/workflows/ci.yml

@@ -21,7 +21,7 @@ env:
   MAKEFLAGS: '-j4'
   BUILD: 'check'
   ### secp256k1 config
-  ECMULTWINDOW: 'auto'
+  ECMULTWINDOW: 15
   ECMULTGENKB: 'auto'
   ASM: 'no'
   WIDEMUL: 'auto'

configure.ac

@@ -203,15 +203,15 @@ AC_ARG_WITH([test-override-wide-multiply], [] ,[set_widemul=$withval], [set_wide
 AC_ARG_WITH([asm], [AS_HELP_STRING([--with-asm=x86_64|arm32|no|auto],
 [assembly to use (experimental: arm32) [default=auto]])],[req_asm=$withval], [req_asm=auto])
 
-AC_ARG_WITH([ecmult-window], [AS_HELP_STRING([--with-ecmult-window=SIZE|auto],
+AC_ARG_WITH([ecmult-window], [AS_HELP_STRING([--with-ecmult-window=SIZE],
 [window size for ecmult precomputation for verification, specified as integer in range [2..24].]
 [Larger values result in possibly better performance at the cost of an exponentially larger precomputed table.]
 [The table will store 2^(SIZE-1) * 64 bytes of data but can be larger in memory due to platform-specific padding and alignment.]
 [A window size larger than 15 will require you delete the prebuilt precomputed_ecmult.c file so that it can be rebuilt.]
 [For very large window sizes, use "make -j 1" to reduce memory use during compilation.]
-["auto" is a reasonable setting for desktop machines (currently 15). [default=auto]]
+[The default value is a reasonable setting for desktop machines (currently 15). [default=15]]
 )],
-[req_ecmult_window=$withval], [req_ecmult_window=auto])
+[set_ecmult_window=$withval], [set_ecmult_window=15])
 
 AC_ARG_WITH([ecmult-gen-kb], [AS_HELP_STRING([--with-ecmult-gen-kb=2|22|86|auto],
 [The size of the precomputed table for signing in multiples of 1024 bytes (on typical platforms).]
@@ -335,14 +335,7 @@ auto)
   ;;
 esac
 
-# Set ecmult window size
-if test x"$req_ecmult_window" = x"auto"; then
-  set_ecmult_window=15
-else
-  set_ecmult_window=$req_ecmult_window
-fi
-
-error_window_size=['window size for ecmult precomputation not an integer in range [2..24] or "auto"']
+error_window_size=['window size for ecmult precomputation not an integer in range [2..24]']
 case $set_ecmult_window in
 ''|*[[!0-9]]*)
   # no valid integer