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 and do not rely on using the system default compilers,
and use the gcc and g++ compilers explicitly.

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

@@ -681,8 +681,5 @@
 		else
 			version = ""
 		end
-		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 (cfg.gccprefix or "") .. gcc.tools[tool] .. version
 	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