#define flash const __flash
или
typedef const __flash flash;

1) flash void* flash
2) void* flash
3) flash void*

avr-gcc -v

Using built-in specs.
COLLECT_GCC=c:\gcc\avr-gcc\bin\avr-gcc.EXE
COLLECT_LTO_WRAPPER=c:/gcc/avr-gcc/bin/../libexec/gcc/avr/4.7.1/lto-wrapper.exe
Target: avr
Configured with: ../../gcc.gnu.org/gcc-4_7-branch/configure --target=avr --prefix=/local/gnu/install/gcc-4.7-mingw32 --host=i386-mingw32 --build=i686-linux-gnu --enable-languages=c,c++ --disable-nls --disable-shared --with-dwarf2 : (reconfigured) ../../gcc.gnu.org/gcc-4_7-branch/configure --target=avr --prefix=/local/gnu/install/gcc-4.7-mingw32 --host=i386-mingw32 --build=i686-linux-gnu --enable-languages=c,c++ --disable-nls --disable-shared --with-dwarf2
Thread model: single
gcc version 4.7.1 20120606 (prerelease) (GCC)

Binutils 2.22.52.20120702 (development snapshot)

#CFLAGS += -flto
#CFLAGS += -mbranch-cost=
CFLAGS += -fno-tree-switch-conversion
CFLAGS += -mstrict-X
CFLAGS += -maccumulate-args

const __flash char * pcf1;
const char * pc2;

void test1(void)
{
    pcf1 = pc2;
}

void test2(void)
{
    pc2 = pcf1;
}