compilers: Move lto args into compiler class

There are two problems, one is that it assumes -flto is the argument
to do LTO/WPO, which isn't true of ICC and MSVC (and presumably)
others. It's also incorrect because it assumes that the compiler and
linker will always be the same, which isn't necessarily true. You
could combine GCC with Apple's linker, or clang with link.exe, which
use different arguments.

mesonbuild/compilers/compilers.py

@@ -13,7 +13,7 @@
 # limitations under the License.
 
 import contextlib, enum, os.path, re, tempfile, shlex
-from typing import Optional, Tuple
+from typing import Optional, Tuple, List
 
 from ..linkers import StaticLinker
 from .. import coredata
@@ -278,10 +278,9 @@ def option_enabled(boptions, options, option):
 
 def get_base_compile_args(options, compiler):
     args = []
-    # FIXME, gcc/clang specific.
     try:
         if options['b_lto'].value:
-            args.append('-flto')
+            args.extend(compiler.get_lto_compile_args())
     except KeyError:
         pass
     try:
@@ -328,10 +327,9 @@ def get_base_compile_args(options, compiler):
 
 def get_base_link_args(options, linker, is_shared_module):
     args = []
-    # FIXME, gcc/clang specific.
     try:
         if options['b_lto'].value:
-            args.append('-flto')
+            args.extend(linker.get_lto_link_args())
     except KeyError:
         pass
     try:
@@ -1197,6 +1195,12 @@ class Compiler:
     def remove_linkerlike_args(self, args):
         return [x for x in args if not x.startswith('-Wl')]
 
+    def get_lto_compile_args(self) -> List[str]:
+        return []
+
+    def get_lto_link_args(self) -> List[str]:
+        return []
+
 
 @enum.unique
 class CompilerType(enum.Enum):

mesonbuild/compilers/mixins/gnu.py

@@ -366,6 +366,12 @@ class GnuLikeCompiler(metaclass=abc.ABCMeta):
                 return self._split_fetch_real_dirs(line.split('=', 1)[1])
         return []
 
+    def get_lto_compile_args(self) -> typing.List[str]:
+        return ['-flto']
+
+    def get_lto_link_args(self) -> typing.List[str]:
+        return ['-flto']
+
 
 class GnuCompiler(GnuLikeCompiler):
     """