Use cdecl calling conventions on Win32

The recommendation from engineers within Microsoft is that libraries
should have a calling convention specified in the public API, and that
calling convention should be cdecl unless there are strong reasons to
use a different calling convention.

We previously offered end-users the choice between cdecl and stdcall
calling conventions.  We did this for presumed wider compatibility: most
Windows applications will use cdecl, but C# and PInvoke default to
stdcall for WINAPI compatibility.  (On Windows, the standard library
functions are are stdcall so PInvoke also defaults to stdcall.)

However, C# and PInvoke can easily call cdecl APIs by specifying an
annotation.

Thus, we will explicitly declare ourselves cdecl and remove the option
to build as stdcall.

CMakeLists.txt

@@ -74,14 +74,6 @@ IF (APPLE)
 ENDIF()
 
 IF(MSVC)
-	# This option is only available when building with MSVC. By default, libgit2
-	# is build using the cdecl calling convention, which is useful if you're
-	# writing C. However, the CLR and Win32 API both expect stdcall.
-	#
-	# If you are writing a CLR program and want to link to libgit2, you'll want
-	# to turn this on by invoking CMake with the "-DSTDCALL=ON" argument.
-	OPTION(STDCALL			"Build libgit2 with the __stdcall convention"		OFF)
-
 	# This option must match the settings used in your program, in particular if you
 	# are linking statically
 	OPTION(STATIC_CRT		"Link the static CRT libraries"		 		 ON)
@@ -125,10 +117,8 @@ IF (MSVC)
 	# /MP - Parallel build
 	SET(CMAKE_C_FLAGS "/GF /MP /nologo ${CMAKE_C_FLAGS}")
 
-	IF (STDCALL)
-		# /Gz - stdcall calling convention
-		SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} /Gz")
-	ENDIF ()
+	# /Gd - explicitly set cdecl calling convention
+	SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} /Gd")
 
 	IF (STATIC_CRT)
 		SET(CRT_FLAG_DEBUG "/MTd")

README.md

@@ -255,7 +255,6 @@ The following CMake variables are declared:
 - `BUILD_SHARED_LIBS`: Build libgit2 as a Shared Library (defaults to ON)
 - `BUILD_CLAR`: Build [Clar](https://github.com/vmg/clar)-based test suite (defaults to ON)
 - `THREADSAFE`: Build libgit2 with threading support (defaults to ON)
-- `STDCALL`: Build libgit2 as `stdcall`. Turn off for `cdecl` (Windows; defaults to ON)
 
 To list all build options and their current value, you can do the
 following:

include/git2/common.h

@@ -43,7 +43,7 @@ typedef size_t size_t;
 			 __attribute__((visibility("default"))) \
 			 type
 #elif defined(_MSC_VER)
-# define GIT_EXTERN(type) __declspec(dllexport) type
+# define GIT_EXTERN(type) __declspec(dllexport) type __cdecl
 #else
 # define GIT_EXTERN(type) extern type
 #endif