Fix gmake action mixing up compilers from different toolsets.

Due to Make behavior, specifically its default values to the CC and CXX
implicit rules, we could end up in a scenario where the gcc and clang
compilers would be used in the same project.

Be more explicit into using either the `cc` or `c++` system default
compilers when no toolset is specified, and `gcc` and `g++` compilers
when the `gcc` toolset is specified.

Fixes #2207.

modules/gmake/tests/cpp/test_tools.lua

@@ -30,6 +30,15 @@
 	function suite.usesCorrectTools()
 		make.cppTools(cfg, p.tools.gcc)
 		test.capture [[
+  ifeq ($(origin CC), default)
+    CC = cc
+  endif
+  ifeq ($(origin CXX), default)
+    CXX = c++
+  endif
+  ifeq ($(origin AR), default)
+    AR = ar
+  endif
   RESCOMP = windres
 		]]
 	end

modules/gmake2/tests/test_gmake2_tools.lua

@@ -31,6 +31,15 @@
 	function suite.usesCorrectTools()
 		gmake2.cpp.tools(cfg, p.tools.gcc)
 		test.capture [[
+ifeq ($(origin CC), default)
+  CC = cc
+endif
+ifeq ($(origin CXX), default)
+  CXX = c++
+endif
+ifeq ($(origin AR), default)
+  AR = ar
+endif
 RESCOMP = windres
 		]]
 	end

src/tools/gcc.lua

@@ -505,14 +505,16 @@
 		architecture = {
 			x86 = function (cfg)
 				local r = {}
-				if not table.contains(os.getSystemTags(cfg.system), "darwin") then
+				if not (table.contains(os.getSystemTags(cfg.system), "darwin")
+					or table.contains(os.getSystemTags(cfg.system), "emscripten")) then
 					table.insert (r, "-L/usr/lib32")
 				end
 				return r
 			end,
 			x86_64 = function (cfg)
 				local r = {}
-				if not table.contains(os.getSystemTags(cfg.system), "darwin") then
+				if not (table.contains(os.getSystemTags(cfg.system), "darwin")
+					or table.contains(os.getSystemTags(cfg.system), "emscripten")) then
 					table.insert (r, "-L/usr/lib64")
 				end
 				return r
@@ -667,6 +669,13 @@
 --    default value should be used.
 --
 
+	gcc.systemtools = {
+		cc = "cc",
+		cxx = "c++",
+		ar = "ar",
+		rc = "windres"
+	}
+
 	gcc.tools = {
 		cc = "gcc",
 		cxx = "g++",
@@ -684,5 +693,9 @@
 		if ((cfg.gccprefix  or version ~= "") and gcc.tools[tool]) or tool == "rc" then
 			return (cfg.gccprefix or "") .. gcc.tools[tool] .. version
 		end
-		return nil
+		-- if no toolset was specified, then lets default to system tools
+		if cfg.toolset == nil then
+			return gcc.systemtools[tool]
+		end
+		return gcc.tools[tool]
 	end

tests/tools/test_gcc.lua

@@ -33,9 +33,18 @@
 
 	function suite.tools_onDefaults()
 		prepare()
-		test.isnil(gcc.gettoolname(cfg, "cc"))
-		test.isnil(gcc.gettoolname(cfg, "cxx"))
-		test.isnil(gcc.gettoolname(cfg, "ar"))
+		test.isequal("cc", gcc.gettoolname(cfg, "cc"))
+		test.isequal("c++", gcc.gettoolname(cfg, "cxx"))
+		test.isequal("ar", gcc.gettoolname(cfg, "ar"))
+		test.isequal("windres", gcc.gettoolname(cfg, "rc"))
+	end
+
+	function suite.tools_withGcc()
+		toolset "gcc"
+		prepare()
+		test.isequal("gcc", gcc.gettoolname(cfg, "cc"))
+		test.isequal("g++", gcc.gettoolname(cfg, "cxx"))
+		test.isequal("ar", gcc.gettoolname(cfg, "ar"))
 		test.isequal("windres", gcc.gettoolname(cfg, "rc"))
 	end