[latexila] build: document why adl_RECURSIVE_EVAL is still needed

[Sébastien Wilmet <swilmet src gnome org>]

commit d13f272c554dd85802a4a5670f3aa180b9607e5d
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Fri Nov 3 16:12:02 2017 +0100

    build: document why adl_RECURSIVE_EVAL is still needed
    
    I wanted to remove this ugly code, but it's not yet possible.
    
    (BTW it's not me who wrote the macro, I just copied it from somewhere
    else, but I don't remember where).

 configure.ac |   11 +++++++++++
 1 files changed, 11 insertions(+), 0 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index a441769..0420287 100644
--- a/configure.ac
+++ b/configure.ac
@@ -54,6 +54,17 @@ AC_SUBST([GEE_REQUIRED_VERSION])
 # WARNING: It's easy to get an infinite loop with some unsane input.
 # For example ${datadir} becomes ${datarootdir}, and then ${prefix}/share, and
 # finally ${prefix} is replaced by the prefix.
+#
+# Why is it used in LaTeXila?
+# TL;DR: Vala.
+#
+# If the variable is needed only in C code, it is possible to pass the value
+# with -D in the CPPFLAGS in the Makefile.am, and the value will correctly be
+# interpolated.
+# This macro is used in LaTeXila because the variables are needed in Vala, the
+# variables are accessed through vapi/config.vapi, so they need to be present
+# in the config.h (the variables are added to the config.h with
+# AC_DEFINE_UNQUOTED).
 AC_DEFUN([adl_RECURSIVE_EVAL],
 [_lcl_receval="$1"
 $2=`(test "x$prefix" = xNONE && prefix="$ac_default_prefix"