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`.

1 files changed, 11 insertions, 0 deletions

diff --git a/cmake/ExperimentalFeatures.cmake b/cmake/ExperimentalFeatures.cmake
index f23bfe376..57a66e68d 100644
--- a/cmake/ExperimentalFeatures.cmake
+++ b/cmake/ExperimentalFeatures.cmake
@@ -1,8 +1,19 @@
+# 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()