Check if alloca uses alloca.h

Check this because alloca() in Solaris needs alloca.h and Win32 doesn't
have the header.

configure.ac

@@ -58,7 +58,7 @@ XMP_TRY_COMPILE(whether compiler understands -fvisibility=hidden,
   ac_cv_c_flag_f_visibility_hidden,
   -fvisibility=hidden,[
   int main(){}],
-  [CFLAGS="${CFLAGS} -fvisibility=hidden"
+  [LDFLAGS="${LDFLAGS} -fvisibility=hidden"
   LD_VERSCRIPT="-Wl,--version-script,libxmp.map"]) 
 
 XMP_TRY_COMPILE(whether compiler understands -Wunused-but-set-variable,
@@ -79,6 +79,12 @@ XMP_TRY_COMPILE(whether compiler understands -Warray-bounds,
   int main(){}],
   CFLAGS="${CFLAGS} -Wno-array-bounds")  
 
+XMP_TRY_COMPILE(whether alloca() needs alloca.h,
+  ac_cv_c_flag_w_have_alloca_h,,[
+  #include <alloca.h>
+  int main(){}],
+  AC_DEFINE(HAVE_ALLOCA_H))  
+
 AC_CHECK_LIB(m,pow)
 AC_CHECK_FUNCS(popen mkstemp fnmatch strlcpy)
 AC_CONFIG_FILES([Makefile])

src/vorbis.h

@@ -42,6 +42,10 @@ extern "C" {
 // will occur from it as described above. Otherwise just pass NULL
 // to use malloc()/alloca()
 
+#ifdef HAVE_ALLOCA_H
+#include <alloca.h>
+#endif
+
 typedef struct
 {
    char *alloc_buffer;