[gspell] build: add note about builddir != srcdir issue

[Sébastien Wilmet <swilmet src gnome org>]

commit 84a0b89c9641be3ff43953b365848856a6664b4a
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Sun Mar 13 19:40:43 2016 +0100

    build: add note about builddir != srcdir issue
    
    See previous commit.

 configure.ac |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 0591764..a2f7dc3 100644
--- a/configure.ac
+++ b/configure.ac
@@ -41,6 +41,7 @@ GSPELL_LT_VERSION="$GSPELL_LT_CURRENT:$GSPELL_LT_REVISION:$GSPELL_LT_AGE"
 AC_SUBST(GSPELL_LT_VERSION)
 
 # API version, used for parallel installability.
+# Not used in po/, it causes a builddir != srcdir build issue.
 GSPELL_API_VERSION=1
 AC_SUBST(GSPELL_API_VERSION)