Improved variable length array support.

VLA storage is now freed when it goes out of scope. This makes it
possible to use a VLA inside a loop without consuming an unlimited
amount of memory.

Combining VLAs with alloca() should work as in GCC - when a VLA is
freed, memory allocated by alloca() after the VLA was created is also
freed. There are some exceptions to this rule when using goto: if a VLA
is in scope at the goto, jumping to a label will reset the stack pointer
to where it was immediately after the last VLA was created prior to the
label, or to what it was before the first VLA was created if the label
is outside the scope of any VLA. This means that in some cases combining
alloca() and VLAs will free alloca() memory where GCC would not.

configure

@@ -19,6 +19,7 @@ TMPH=$TMPN.h
 # default parameters
 build_cross="no"
 use_libgcc="no"
+enable_assert="no"
 prefix=""
 execprefix=""
 bindir=""
@@ -165,6 +166,8 @@ for opt do
   ;;
   --enable-cross) build_cross="yes"
   ;;
+  --enable-assert) enable_assert="yes"
+  ;;
   --disable-static) disable_static="yes"
   ;;
   --disable-rpath) disable_rpath="yes"
@@ -274,6 +277,7 @@ Advanced options (experts only):
   --enable-mingw32         build windows version on linux with mingw32
   --enable-cygwin          build windows version on windows with cygwin
   --enable-cross           build cross compilers
+  --enable-assert          enable debug assertions
   --with-selinux           use mmap for exec mem [needs writable /tmp]
   --sysincludepaths=...    specify system include paths, colon separated
   --libpaths=...           specify system library paths, colon separated
@@ -484,6 +488,9 @@ if test "$have_selinux" = "yes" ; then
   echo "#define HAVE_SELINUX" >> $TMPH
   echo "HAVE_SELINUX=yes" >> config.mak
 fi
+if test "$enable_assert" = "yes" ; then
+  echo "#define CONFIG_TCC_ASSERT" >> $TMPH
+fi
 
 version=`head $source_path/VERSION`
 echo "VERSION=$version" >>config.mak