add --disable-alloca to configure

It looks like alloca() is broken on mingw64, and thus
we need to disable it explicitly.

configure.ac

@@ -1,5 +1,5 @@
 dnl Process this file with autoconf to produce a configure script.
-AC_INIT(fftw, 3.2.1, [email protected])
+AC_INIT(fftw, 3.2.2alpha, [email protected])
 AC_CONFIG_SRCDIR(kernel/ifftw.h)
 SHARED_VERSION_INFO="5:3:2" # CURRENT:REVISION:AGE
 
@@ -44,6 +44,11 @@ if test "$ok" = "yes"; then
 	AC_DEFINE(FFTW_DEBUG_ALIGNMENT,1,[Define to enable alignment debugging hacks.])
 fi
 
+AC_ARG_ENABLE(alloca, [AC_HELP_STRING([--enable-alloca],[enable use of the alloca() function (may be broken on mingw64)])], ok=$enableval, ok=yes)
+if test "$ok" = "yes"; then
+	AC_DEFINE(FFTW_ENABLE_ALLOCA,1,[Define to enable the use of alloca().])
+fi
+
 AC_ARG_ENABLE(single, [AC_HELP_STRING([--enable-single],[compile fftw in single precision])], ok=$enableval, ok=no)
 AC_ARG_ENABLE(float, [AC_HELP_STRING([--enable-float],[synonym for --enable-single])], ok=$enableval)
 if test "$ok" = "yes"; then

kernel/ifftw.h

@@ -107,7 +107,7 @@ typedef struct scanner_s scanner;
 #define MIN_ALIGNMENT 16
 #endif
 
-#ifdef HAVE_ALLOCA
+#if defined(HAVE_ALLOCA) && defined(FFTW_ENABLE_ALLOCA)
    /* use alloca if available */
 
 #ifndef alloca