src/sage/libs/giac/meson.build: build extension only if libgiac is found

The giac package is already marked optional, but it isn't really
optional because the cython extension can't be built without libgiac.
Now, the extension will be skipped if giac is absent. Recent changes
in sagelib have made this feasible.

src/sage/libs/giac/meson.build

@@ -4,15 +4,16 @@ py.install_sources('__init__.py', 'giac.pxd', 'misc.h', subdir: 'sage/libs/giac'
 
 extension_data_cpp = {'giac': files('giac.pyx')}
 
-foreach name, pyx : extension_data_cpp
-  py.extension_module(
-    name,
-    sources: pyx,
-    subdir: 'sage/libs/giac',
-    install: true,
-    override_options: ['cython_language=cpp'],
-    include_directories: [inc_cpython, inc_ext, inc_rings],
-    dependencies: [py_dep, cysignals, giac, gmp],
-  )
-endforeach
-
+if giac.found()
+  foreach name, pyx : extension_data_cpp
+    py.extension_module(
+      name,
+      sources: pyx,
+      subdir: 'sage/libs/giac',
+      install: true,
+      override_options: ['cython_language=cpp'],
+      include_directories: [inc_cpython, inc_ext, inc_rings],
+      dependencies: [py_dep, cysignals, giac, gmp],
+    )
+  endforeach
+endif