[cairo.h] Be defensive when checking for _MSC_VER

Since cairo.h is a public header file, we need to be careful so that it
can be compiled by random compilers and even users specifying "-Werror
-Wundef" (mentioning no names, Company). So replace the bare (and legal)
  #if _MSC_VER
with
  #if defined (_MSC_VER)
just in case.

src/cairo.h

@@ -51,7 +51,7 @@
 #endif
 
 #ifndef cairo_public
-# if _MSC_VER && !CAIRO_WIN32_STATIC_BUILD
+# if defined (_MSC_VER) && ! defined (CAIRO_WIN32_STATIC_BUILD)
 #  define cairo_public __declspec(dllimport)
 # else
 #  define cairo_public