config: refactor and comment the sdkdir distcheck workaround

Seperate legitimate sdkdir usage from distcheck workaround.
Comment this non obvious workaround.

Signed-off-by: Gaetan Nadon <memsize@videotron.ca>

Makefile.am

@@ -19,7 +19,7 @@
 #  CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 
 
-# Ensure headers are installed below $(prefix) for distcheck
+# Provide an sdk location that is writable by the evdev module
 DISTCHECK_CONFIGURE_FLAGS = --with-sdkdir='$${includedir}/xorg'
 
 SUBDIRS = src man include

configure.ac

@@ -60,13 +60,17 @@ XORG_DRIVER_CHECK_EXT(XINPUT, inputproto)
 # to satisfy silly distcheck requirements.
 PKG_CHECK_MODULES(XORG, xorg-server xproto $REQUIRED_MODULES)
 
-AC_ARG_WITH([sdkdir], [],
-    [sdkdir="$withval"],
-    [sdkdir=`$PKG_CONFIG --variable=sdkdir xorg-server`])
-AC_SUBST([sdkdir])
-
 # Checks for libraries.
 
+# X Server SDK location is required to install evdev header files
+# This location is also relayed in the xorg-evdev.pc file
+sdkdir=`$PKG_CONFIG --variable=sdkdir xorg-server`
+
+# Workaround overriding sdkdir to be able to create a tarball when user has no
+# write permission in sdkdir. See DISTCHECK_CONFIGURE_FLAGS in Makefile.am
+AC_ARG_WITH([sdkdir], [], [sdkdir="$withval"])
+AC_SUBST([sdkdir])
+
 DRIVER_NAME=evdev
 AC_SUBST([DRIVER_NAME])