Wrong prefix checking of global variables

[Pavel Kopyl]

Hello,

I faced with the situation where ./configure makes wrong decision about 
if global variables should be prefixed or not.

Conditions.

Version:             trunk (GNU MP 6.1.99)
Host type:         armv7l-unknown-linux-gnueabi
Compiler:          Linaro GCC 4.9.2

./configure \
     --prefix=/usr \
     --includedir=/usr/include \
     --libdir=/usr/lib \
     --enable-cxx \


To check if a global variable should be prefixed with underscore 
./configure "greps" variable's name in object file in the following test:

cat conftest.c
int gurkmacka;


cat configure
..
$NM conftest.$OBJEXT >conftest.out
   if grep _gurkmacka conftest.out >/dev/null; then
     gmp_cv_asm_underscore=yes
   elif grep gurkmacka conftest.out >/dev/null; then
     gmp_cv_asm_underscore=no
   else
     echo "configure: $NM doesn't have gurkmacka:" >&5
     cat conftest.out >&5
...

Generally it works fine, but being built with enabled AddressSanitizer 
(-fsanitize=address, -fno-common), compiler generates instrumented code and
object file contains additional symbols whose names may incorporate 
"_gurkmacka" as a substring, though actually global variable is not 
prefixed.

Lets see an example:

Without AddressSanitizer:

cat conftest.out
00000004 C gurkmacka


With AddressSanitizer:

cat conftest.out
00000000 t _GLOBAL__sub_D_00099_0_gurkmacka
00000000 t _GLOBAL__sub_I_00099_1_gurkmacka
          U __aeabi_unwind_cpp_pr0
          U __asan_init
          U __asan_register_globals
          U __asan_unregister_globals
          U __asan_version_mismatch_check_v6
00000000 B gurkmacka

Based on the former example ./configure erroneously assumes that global 
variables should be prefixed.**

Attached patch fixes this issue.


-Pavel





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