Fix release builds with Clang and SCons

The detection for when Clang is used was incorrect, resulting in GCC-only
parameter being passed to Clang.

In addition, at least on my system LTO+Clang caused linking errors even
with that fixed. As an emergency fix, I completely disabled LTO for Clang.

SConstruct

@@ -525,7 +525,7 @@ for env in [test_env, client_env, env]:
         rel_comp_flags = rel_comp_flags + env["arch"]
 
 # PGO and LTO setup
-        if "gcc" in env["TOOLS"]:
+        if "gcc" in env["CXX"]:
             if env["pgo_data"] == "generate":
                 rel_comp_flags = rel_comp_flags + " -fprofile-generate=pgo_data/"
                 rel_link_flags = "-fprofile-generate=pgo_data/"
@@ -537,6 +537,9 @@ for env in [test_env, client_env, env]:
             if env["enable_lto"] == True:
                 rel_comp_flags = rel_comp_flags + " -flto=" + str(env["jobs"])
                 rel_link_flags = rel_comp_flags + " -fuse-ld=gold"
+        ''' Disabled because LTO+Clang causes builds to fail on some systems
+        and LTO is enabled by default - without this, building with Clang would be impossible
+        for some users.
         elif "clang" in env["CXX"]:
             if env["pgo_data"] == "generate":
                 rel_comp_flags = rel_comp_flags + " -fprofile-instr-generate=pgo_data/wesnoth-%p.profraw"
@@ -549,6 +552,7 @@ for env in [test_env, client_env, env]:
             if env["enable_lto"] == True:
                 rel_comp_flags = rel_comp_flags + " -flto=thin"
                 rel_link_flags = rel_comp_flags + " -fuse-ld=lld"
+        '''
 
 # #
 # End setting options for release build