Specifiy dllimport to MSVC if we're not building libgit2.dll

Building a "shared object" (DLL) in Windows includes 2 steps:

- specify __declspec(dllexport)
  when building the library itself. MSVC will disallow itself from
  optimizing these symbols out and reference them in the PE's
  Exports-Table.
  Further, a static link library will be generated. This library
  contains the symbols which are exported via the declsepc above.
  The __declspec(dllexport) becomes part of the symbol-signature
  (like parameter types in C++ are 'mangled' into the symbol name,
  the export specifier is mingled with the name)

- specify __declspec(dllimport)
  when using the library. This again mingles the declspec into the
  name and declares the function / variable with external linkage.

cmake automatically adds -Dgit2_EXPORTS to the compiler arguments
when compiling the libgit2 project.
The 'git2' is the name specified via PROJECT() in CMakeLists.txt.

include/git2/common.h

@@ -32,7 +32,11 @@
 			 __attribute__((visibility("default"))) \
 			 type
 #elif defined(_MSC_VER)
+# ifdef git2_EXPORTS /* defined by cmake */
 #   define GIT_EXTERN(type) __declspec(dllexport) type
+# else
+#   define GIT_EXTERN(type) __declspec(dllimport) type
+# endif
 #else
 # define GIT_EXTERN(type) extern type
 #endif