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.

modules/gmake/tests/cpp/test_tools.lua

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

src/tools/gcc.lua

@@ -684,5 +684,5 @@
 		if ((cfg.gccprefix  or version ~= "") and gcc.tools[tool]) or tool == "rc" then
 			return (cfg.gccprefix or "") .. gcc.tools[tool] .. version
 		end
-		return nil
+		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("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
+
+	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