errors building gcc-10.2

[Ron Zamir <zamir.ron@gmail.com>]

trying to build gcc-10.2
I've added the latest sources for the GMP, MPFR, MPC, ISL libraries
My system is CentOS 7.8 with gcc version 4.85

I'm running configure with the following parameters:
CC=/usr/bin/gcc CXX=/usr/bin/g++ ../gcc-10.2.0/configure
CFLAGS="-std=gnu99" CXXFLAGS="-std=c++11" --prefix=$HOME/.local/gcc-10
--disable-multilib

the additional CFLAGS and CXXFLAGS parameters where added to try to fix the
error (below)

when I run make, I get the error:
libtool: compile:  /usr/bin/gcc -DHAVE_CONFIG_H -I.
-I../../../gcc-10.2.0/gmp/mpn -I.. -D__GMP_WITHIN_GMP
-I../../../gcc-10.2.0/gmp -DOPERATION_compute_powtab -DNO_ASM -g -c
compute_powtab.c -o compute_powtab.o
compute_powtab.c: In function 'mpn_compute_powtab_mul':
compute_powtab.c:142:3: error: 'for' loop initial declarations are only
allowed in C99 mode
   for (long pi = start_idx; pi >= 0; pi--)
   ^
compute_powtab.c:142:3: note: use option -std=c99 or -std=gnu99 to compile
your code
compute_powtab.c: In function 'mpn_compute_powtab_div':
compute_powtab.c:226:3: error: 'for' loop initial declarations are only
allowed in C99 mode
   for (long pi = n_pows - 1; pi >= 0; pi--)
   ^
compute_powtab.c:274:13: error: redefinition of 'pi'
   for (long pi = n_pows; pi >= 0; pi--)
             ^
compute_powtab.c:226:13: note: previous definition of 'pi' was here
   for (long pi = n_pows - 1; pi >= 0; pi--)
             ^
compute_powtab.c:274:3: error: 'for' loop initial declarations are only
allowed in C99 mode
   for (long pi = n_pows; pi >= 0; pi--)
   ^
compute_powtab.c: In function 'powtab_decide':
compute_powtab.c:296:3: error: 'for' loop initial declarations are only
allowed in C99 mode
   for (size_t pn = (un + 1) >> 1; pn != 1; pn = (pn + 1) >> 1)
   ^
compute_powtab.c:304:10: error: redefinition of 'pn'
   size_t pn = un - 1;
          ^
compute_powtab.c:296:15: note: previous definition of 'pn' was here
   for (size_t pn = (un + 1) >> 1; pn != 1; pn = (pn + 1) >> 1)
               ^
compute_powtab.c:308:3: error: 'for' loop initial declarations are only
allowed in C99 mode
   for (long i = n_pows - 2; i >= 0; i--)
   ^
make[5]: *** [compute_powtab.lo] Error 1

what flags should I specify to fix this error?