Simplify symbol export

Currently, the logic for exporting symbols is rather complicated: when
tests are enabled internal symbols are exported in addition to the
public symbols. Such logic causes several problems. (1) Test binaries
link against a Ceres build that is different from the final release
since fewer optimizations are applied if more symbols are exported. (2)
Also, some toolchains hide symbols by default breaking the existing
logic eventually causing linker errors.

Since internal symbols are not intended to be used outside of the
project, we can compile them into object files and use exactly the same
binary code both for the final build and the tests without relying on
conditionals.

By default, all symbols are now hidden unless annotated as public.
Internal symbols are explicitly marked as not being exported in case
users chose not to hide symbols by default.

Change-Id: I589dd10be2f6f438508783cf99d141af0120057b
diff --git a/internal/ceres/parallel_for_test.cc b/internal/ceres/parallel_for_test.cc
index 434f993..b6eb5dc 100644
--- a/internal/ceres/parallel_for_test.cc
+++ b/internal/ceres/parallel_for_test.cc
@@ -30,7 +30,7 @@
 
 // This include must come before any #ifndef check on Ceres compile options.
 // clang-format off
-#include "ceres/internal/port.h"
+#include "ceres/internal/config.h"
 // clang-format on
 
 #include "ceres/parallel_for.h"