Tests: Use consistent C++ flags FindPackageModeMakefileTest

Rather than using the CXXFLAGS environment variable in the make-only
build, copy the CMAKE_CXX_FLAGS used to build the files on the CMake
side.  This will account for any changes made by CompileFlags.cmake
or cache-provided flags.

Tests/FindPackageModeMakefileTest/Makefile.in

@@ -4,6 +4,7 @@ include cmakeExecutable.mk
 CMAKE_CURRENT_BINARY_DIR = "@CMAKE_CURRENT_BINARY_DIR@"
 CMAKE_CXX_COMPILER = "@CMAKE_CXX_COMPILER@"
 CMAKE_CXX_COMPILER_ID = "@CMAKE_CXX_COMPILER_ID@"
+CMAKE_CXX_FLAGS = @CMAKE_CXX_FLAGS@
 
 CMAKE_FOO = $(CMAKE) --find-package -DCMAKE_MODULE_PATH=$(CMAKE_CURRENT_BINARY_DIR) -DNAME=Foo -DLANGUAGE=CXX -DCOMPILER_ID=$(CMAKE_CXX_COMPILER_ID)
 
@@ -14,7 +15,7 @@ all: pngtest
 main.o: clean main.cpp
 	@$(CMAKE_FOO) -DMODE=COMPILE >$(tmp)
 	@foo="`cat $(tmp)`"; \
-	 printf '"%s" %s %s -c main.cpp\n' $(CMAKE_CXX_COMPILER) "$(CXXFLAGS)" "$$foo" >$(tmp)
+	 printf '"%s" %s %s -c main.cpp\n' $(CMAKE_CXX_COMPILER) "$(CMAKE_CXX_FLAGS)" "$$foo" >$(tmp)
 	@cat $(tmp)
 	@sh $(tmp)
 	@rm -f $(tmp)
@@ -22,7 +23,7 @@ main.o: clean main.cpp
 pngtest: main.o
 	@$(CMAKE_FOO) -DMODE=LINK >$(tmp)
 	@foo="`cat $(tmp)`"; \
-	 printf '"%s" %s %s -o pngtest main.o %s\n' $(CMAKE_CXX_COMPILER) "$(CXXFLAGS)" "$(LDFLAGS)" "$$foo" >$(tmp)
+	 printf '"%s" %s %s -o pngtest main.o %s\n' $(CMAKE_CXX_COMPILER) "$(CMAKE_CXX_FLAGS)" "$(LDFLAGS)" "$$foo" >$(tmp)
 	@cat $(tmp)
 	@sh $(tmp)
 	@rm -f $(tmp)