cmake: don't disable _FORTIFY_SOURCE

[jzern]

This was inherited from libaom, which inherited it from libvpx, where it
was added long ago as an attempt for compatibility across glibc
versions. Formalizing it in the build was never a great idea as this
provides some security benefits. Projects like Gentoo have removed this
from the build flags for some time.

CFLAGS/AVM_EXTRA_C_FLAGS can still be used to set these values if
needed.

This matches the changes in libvpx and libaom:
368c5f89d configure: don't disable _FORTIFY_SOURCE
ec0dedc1a2 cmake: don't disable _FORTIFY_SOURCE

[jzern]

The build failure looks like a preexisting condition:

../avm_dsp/grain_table.c: In function 'grain_table_entry_read':
../avm_dsp/grain_table.c:131:11: error: ignoring return value of 'fscanf' declared with attribute 'warn_unused_result' [-Werror=unused-result]
  131 |     (void)fscanf(file, "\n");
      |

I made some updates to this file in libaom recently to clear some clang-19 static analysis warnings, but I didn't get to this particular call.

[wantehchang]

James: I remember your CL for the aom_dsp/grain_table.c file in libaom and we discussed that (void)fscanf(file, "\n") call.

[jzern]

James: I remember your CL for the aom_dsp/grain_table.c file in libaom and we discussed that (void)fscanf(file, "\n") call.

You're right. The comments have the detail about the last remaining issues in this file. From what I remember more checks than were necessary were needed to satisfy the static analyzer, but I didn't keep the patch around.

aom_dsp/grain_table.c:287:15: warning: File position of the stream might be 'indeterminate' after a failed operation. Can cause undefined behavior [unix.Stream]
  287 |       int c = fgetc(file);
      |               ^~~~~~~~~~~
aom_dsp/grain_table.c:287:15: warning: Read function called when stream is in EOF state. Function has no effect [unix.Stream]
  287 |       int c = fgetc(file);
      |               ^~~~~~~~~~~
aom_dsp/grain_table.c:291:7: warning: File position of the stream might be 'indeterminate' after a failed operation. Can cause undefined behavior [unix.Stream]
  291 |       ungetc(c, file);
      |       ^~~~~~~~~~~~~~~

The html report for those errors is in the libaom static analysis job:

• Line 287
• Line 291

Note the last two links may be unstable.