Update makefiles for MSYS2/MinGW-w64

[feiyunw]

Common changes:

• CXX, CXXFLAGS, LDFLAGS, RM, LDFLAGS and LDLIBS are common variables used in GNU make built-in rules.
  CXX defaults to 'g++", which will be OK usually.
  see GNU Make manual 10.3 Variables Used by Implicit Rules.
• Add -I../include, so the ugly #include statements can be rectified in the future.

src/Makefiles/Makefile_mingw:

• Add common macros for Windows DLLs.
• THREADING & THREAD_LINK: replace boost w/ winapi
  Boost may not be installed, while winapi is essential on MinGW.
• Add MINGW32 support.
• Add -flto -fwhole-program
  FYI:
  w/o -flto: DLL size 387,584 bytes
  w/ -flto (-fuse-linker-plugin): DLL size 409,088 bytes
  w/ -flto -fwhole-program: DLL size 296,960 bytes

test/Makefiles/Makefile_mingw:

• Add common macros for Windows exe.
• $(THREAD_LINK): replace boost w/ winapi
• Use .exe suffix for executales.
  MinGW is for building Windows app.
• Add -flto
• Add missing ThreadMgr.cpp to dds_sources.txt

examples/Makefiles/Makefile_mingw:

• Add common macros for Windows exe.
• Add missing .cpp suffix in $(ALL_EXAMPLE_FILES).
• Replace -Wl,--subsystem,windows w/ -mconsole.
• Optimize rules for executables.
• Use .PRECIOUS to preserve .o files.
  See GNU Make manual 10.4 Chains of Implicit Rules.