cmake: provide empty experimental.h for non-cmake users

Not everybody builds libgit2 using cmake; provide an `experimental.h`
with no experiments configured for those that do not. To support this,
we also now create compile definitions for experimental functionality,
to supplant that empty `experimental.h`. cmake will continue to generate
the proper `experimental.h` file for use with `make install`.

cmake/ExperimentalFeatures.cmake

+# Experimental feature support for libgit2 - developers can opt in to
+# experimental functionality, like sha256 support. When experimental
+# functionality is enabled, we set both a cmake flag *and* a compile
+# definition. The cmake flag is used to generate `experimental.h`,
+# which will be installed by a `make install`. But the compile definition
+# is used by the libgit2 sources to detect the functionality at library
+# build time. This allows us to have an in-tree `experimental.h` with
+# *no* experiments enabled. This lets us support users who build without
+# cmake and cannot generate the `experimental.h` file.
+
 if(EXPERIMENTAL_SHA256)
 	add_feature_info("SHA256 API" ON "experimental SHA256 APIs")
 
 	set(EXPERIMENTAL 1)
 	set(GIT_EXPERIMENTAL_SHA256 1)
+	add_compile_definitions(GIT_EXPERIMENTAL_SHA256)
 else()
 	add_feature_info("SHA256 API" OFF "experimental SHA256 APIs")
 endif()

include/git2/experimental.h

+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+
+#ifndef INCLUDE_experimental_h__
+#define INCLUDE_experimental_h__
+
+/*
+ * This file exists to support users who build libgit2 with a bespoke
+ * build system and do not use our cmake configuration. Normally, cmake
+ * will create `experimental.h` from the `experimental.h.in` file and
+ * will include the generated file instead of this one. For non-cmake
+ * users, we bundle this `experimental.h` file which will be used
+ * instead.
+ */
+
+#endif

src/libgit2/experimental.h.in

+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+
 #ifndef INCLUDE_experimental_h__
 #define INCLUDE_experimental_h__