Support building CSFML as static libraries

Some languages can make use of C foreign function interfaces
compiled as static libraries so it's a good idea for CSFML to
support this use case.

include/CSFML/Config.h

@@ -84,6 +84,8 @@
 ////////////////////////////////////////////////////////////
 // Define helpers to create portable import / export macros for each module
 ////////////////////////////////////////////////////////////
+#if !defined(CSFML_STATIC)
+
 #if defined(CSFML_SYSTEM_WINDOWS)
 
 // Windows compilers need specific (and different) keywords for export and import
@@ -104,6 +106,14 @@
 
 #endif
 
+#else
+
+// Static build doesn't need import/export macros
+#define CSFML_API_EXPORT extern "C"
+#define CSFML_API_IMPORT CSFML_EXTERN_C
+
+#endif
+
 ////////////////////////////////////////////////////////////
 // Cross-platform warning for deprecated functions and classes
 //