Better fix for when CFLAGS includes -D_FORTIFY_SOURCE

The module signals.pyx has to be built with fortify disabled, because of
a false positive in one of the `longjmp()`, more precisely in the call
```
cylongjmp(trampoline_setup, 1)
```
which appears in `setup_trampoline()`.

Often distributions will add `-D_FORTIFY_SOURCE=2` to `CFLAGS`, and so
this has to be overridden when compiling `signals.pyx` by adding
`-U_FORTIFY_SOURCE` to the compiler arguments. The former solution,
using `add_project_arguments()` doesn't work since it will add flags
*before* the ones provided in `CFLAGS`.

Instead, add `-U_FORTIFY_SOURCE` to the arguments in the definition of
the extension module `signal`, which will add it to the command line
*after* the ones provided in `CFLAGS`. In addition, fortify is disabled
only for building `signal.pyx`.

meson.build

@@ -19,9 +19,6 @@ is_mingw = cc.get_id()=='gcc' and host_machine.system()=='windows'
 # Set preprocessor macros
 # Disable .c line numbers in exception tracebacks
 add_project_arguments('-DCYTHON_CLINE_IN_TRACEBACK=0', language: 'c')
-# Disable sanity checking in GNU libc
-# This is required because of false positives in the longjmp() check
-add_project_arguments('-U_FORTIFY_SOURCE', language: 'c')
 
 # Platform-specific settings
 if is_cygwin

src/cysignals/meson.build

@@ -31,6 +31,9 @@ foreach name, pyx : extensions
         cython_args: ['-Wextra'],
         dependencies: [py_dep, threads_dep],
         install: true,
-        subdir: 'cysignals'
+        subdir: 'cysignals',
+        # Disable sanity checking in GNU libc
+        # This is required because of false positives in the longjmp() check
+        c_args: name == 'signals' ? ['-U_FORTIFY_SOURCE'] : [],
     )
 endforeach