use config_xxx.h files instead of -DHAVE_XXX on the command line

As the latter does not quite scale, and also source files depending
on the setting/feature did not rebuild in case of a change.

There are intentionally more config_xxx.h files (so autoheader
from autotools is not used), so that a setting change does not force
automatically a rebuild of everything. Running configure does not
touch those config_xxx.h files that have not changed.

There's config/README with a howto.

Change-Id: I6d6e82050ff8faf5364ed246848b8401aca3e0e5

5 files changed, 62 insertions, 0 deletions

diff --git a/config/.gitignore b/config/.gitignore
new file mode 100644
index 000000000000..325a7fd030e4
--- /dev/null
+++ b/config/.gitignore
@@ -0,0 +1,3 @@
+config_global.h
+config_vclplug.h
+config_telepathy.h
diff --git a/config/README b/config/README
new file mode 100644
index 000000000000..fd58b64045bb
--- /dev/null
+++ b/config/README
@@ -0,0 +1,24 @@
+These are configuration files for various features as detected by configure.
+
+Include only those files you need (in order to reduce rebuilds when a setting changes).
+
+Settings here are only C/C++ #define directives, so they apply only to C/C++ source,
+not to Makefiles.
+
+
+
+Adding a new setting:
+=====================
+
+- do AC_DEFINE(HAVE_FOO) in configure.ac when a setting should be set
+- choose the proper config/config_xxx.h file to use
+    - if it is a global setting (such as availability of a compiler feature),
+        use config/config_global.h
+    - otherwise check if there is a matching config/config_hxx.h file
+    - if none matches, add a new one:
+        - add config/config_xxx.h.in here, with just #ifndef include guard
+        - add AC_CONFIG_HEADERS([config/config_xxx.h]) next to the others
+            in configure.ac
+        - add config_hxx.h to config/.gitignore
+- add #undef HAVE_FOO to the config/config_hxx.h , possibly with a comment
+- add #include <config_xxx.h> before any #ifdef HAVE_XXX in a source file
diff --git a/config/config_global.h.in b/config/config_global.h.in
new file mode 100644
index 000000000000..a9eb54f68c34
--- /dev/null
+++ b/config/config_global.h.in
@@ -0,0 +1,14 @@
+/*
+Global configuration file.
+
+Only for settings that apply to every source file and are unlikely to change often,
+such as whether a certain C++11 feature is available.
+
+Do NOT use for settings local to some code or for settings that can change often.
+Any change in this header will cause a rebuild of almost everything.
+
+*/
+
+#undef HAVE_GCC_BUILTIN_ATOMIC
+#undef HAVE_SFINAE_ANONYMOUS_BROKEN
+#undef HAVE_THREADSAFE_STATICS
diff --git a/config/config_telepathy.h.in b/config/config_telepathy.h.in
new file mode 100644
index 000000000000..f8752b5f5336
--- /dev/null
+++ b/config/config_telepathy.h.in
@@ -0,0 +1,6 @@
+#ifndef CONFIG_TELEPATHY
+#define CONFIG_TELEPATHY
+
+#undef ENABLE_TELEPATHY
+
+#endif
diff --git a/config/config_vclplug.h.in b/config/config_vclplug.h.in
new file mode 100644
index 000000000000..46c6bd1aea77
--- /dev/null
+++ b/config/config_vclplug.h.in
@@ -0,0 +1,15 @@
+/*
+
+Settings about which X11 desktops have support enabled.
+
+*/
+
+#ifndef CONFIG_VCLPLUG_H
+#define CONFIG_VCLPLUG_H
+
+#undef ENABLE_GTK
+#undef ENABLE_KDE
+#undef ENABLE_KDE4
+#undef ENABLE_TDE
+
+#endif