Add new flag ENABLE_STATIC_CRT for Windows

This change adds the CMake option;

  ENABLE_STATIC_CRT  Build libnghttp2 against the MS LIBCMT[d]

This avoids linking to msvcrt.lib for binaries to compile (/MT[d])
and link against the static C Runtime libcrt.lib, and
avoiding the msvcrt[d].dll dependency.
Signed-off-by: William A Rowe Jr <wrowe@pivotal.io>
Signed-off-by: Yechiel Kalmenson <ykalmenson@pivotal.io>

CMakeLists.txt

@@ -387,6 +387,15 @@ else()
   )
 endif()
 
+if(ENABLE_STATIC_CRT)
+  foreach(lang C CXX)
+    foreach(suffix "" _DEBUG _MINSIZEREL _RELEASE _RELWITHDEBINFO)
+      set(var "CMAKE_${lang}_FLAGS${suffix}")
+      string(REPLACE "/MD" "/MT" ${var} "${${var}}")
+    endforeach()
+  endforeach()
+endif()
+
 if(ENABLE_DEBUG)
   set(DEBUGBUILD 1)
 endif()

CMakeOptions.txt

@@ -16,6 +16,7 @@ option(ENABLE_FAILMALLOC "Build failmalloc test program" ON)
 option(ENABLE_LIB_ONLY  "Build libnghttp2 only.  This is a short hand for -DENABLE_APP=0 -DENABLE_EXAMPLES=0 -DENABLE_HPACK_TOOLS=0 -DENABLE_PYTHON_BINDINGS=0")
 option(ENABLE_STATIC_LIB "Build libnghttp2 in static mode also")
 option(ENABLE_SHARED_LIB "Build libnghttp2 as a shared library" ON)
+option(ENABLE_STATIC_CRT "Build libnghttp2 against the MS LIBCMT[d]")
 
 option(WITH_LIBXML2     "Use libxml2"
   ${WITH_LIBXML2_DEFAULT})