Revised build configuration for MSVC.

Major changes and rationale:
- /WX: absolutely vital when compiling in C-mode as the compiler is
  incredibly lenient on what is allowed to compile. It allows functions to be
  called without prototypes declared, treating them as functions returning int
  taking an unspecified (read: unrestricted) list of arguments, without any
  type checking! It will simply issue a warning, which is easily overlooked.

  A real example: it will allow you to call ceil(1.75) without first including
  <math.h> causing UB, returning bogus results like 1023 on the machine I
  tested on.

- Release build separate from debug.
  Presently release builds don't exist.  Consequently they are completely
  untested. Many bugs may only manifest themselves in release mode. The current
  configuration sets debug-only flags like /RTC1 which are incompatible with
  optimization (/O2).

  In addition, the Windows build of libgit2 has no optimized version. This
  change resolves this.

- Added checksum generation in image headers. This is so debuggers don't
  complain about checksum mismatches and provides a small amount of consistency
  to binaries.

wscript

@@ -2,11 +2,18 @@ from waflib.Context import Context
 from waflib.Build import BuildContext, CleanContext, \
         InstallContext, UninstallContext
 
+# Unix flags
 CFLAGS_UNIX = ["-O2", "-Wall", "-Wextra"]
-CFLAGS_WIN32 = ['/TC', '/W4', '/RTC1', '/nologo']
-
 CFLAGS_UNIX_DBG = ['-g']
-CFLAGS_WIN32_DBG = ['/Zi', '/DEBUG']
+
+# Windows MSVC flags
+CFLAGS_WIN32_COMMON = ['/TC', '/W4', '/WX', '/nologo', '/Zi']
+CFLAGS_WIN32_RELEASE = ['/O2', '/MD']
+
+# Note: /RTC* cannot be used with optimization on.
+CFLAGS_WIN32_DBG = ['/Od', '/RTC1', '/RTCc', '/DEBUG', '/MDd']
+CFLAGS_WIN32_L = ['/RELEASE']  # used for /both/ debug and release builds.
+                               # sets the module's checksum in the header.
 CFLAGS_WIN32_L_DBG = ['/DEBUG']
 
 ALL_LIBS = ['z', 'crypto', 'pthread']
@@ -43,8 +50,10 @@ def configure(conf):
         conf.env.PLATFORM = 'win32'
 
         if conf.env.CC_NAME == 'msvc':
-            conf.env.CFLAGS = CFLAGS_WIN32 + (CFLAGS_WIN32_DBG if dbg else [])
-            conf.env.LINKFLAGS += CFLAGS_WIN32_L_DBG if dbg else []
+            conf.env.CFLAGS = CFLAGS_WIN32_COMMON + \
+              (CFLAGS_WIN32_DBG if dbg else CFLAGS_WIN32_RELEASE)
+            conf.env.LINKFLAGS += CFLAGS_WIN32_L + \
+              (CFLAGS_WIN32_L_DBG if dbg else [])
             conf.env.DEFINES += ['WIN32', '_DEBUG', '_LIB', 'ZLIB_WINAPI']
             zlib_name = 'zlibwapi'