lc15: fix configure.ac variable substitution causing compile error

When "--enable-lc15bts-phy" is passed to './configure' without specifying
an explicit header file path using "--with-litecell15=", we ended up
generating an empty string as LITECELL15_INCDIR and rendered something
like "-DENABLE_DIRECT_PHY  -I -I../../git/src/osmo-bts-litecell15" as
part of the compiler invocation, where the -I with no argument will hide
the second -I, as the second one is supposed to be the optarg for the
first include.

This in turn made the "#include <lc15_l1_if.h>" fail, when using
separate source and build directories.

This patch fixes the configur script to use $includedir, rather than the
non-existant $incdir as default for LITECELL15_INCDIR

Change-Id: I483e62f8331e7867a92f8055c4d450fdd5288cb6
diff --git a/configure.ac b/configure.ac
index 54c70ca..7b7f9c4 100644
--- a/configure.ac
+++ b/configure.ac
@@ -51,7 +51,7 @@
                                 [enable code for Litecell 1.5 PHY [default=no]]),
                 [enable_lc15bts_phy="$enableval"],[enable_lc15bts_phy="no"])
 AC_ARG_WITH([litecell15], [AS_HELP_STRING([--with-litecell15=INCLUDE_DIR], [Location of the litecell 1.5 API header files])],
-			 [litecell15_incdir="$withval"],[litecell15_incdir="$incdir"])
+			 [litecell15_incdir="$withval"],[litecell15_incdir="$includedir"])
 AC_SUBST([LITECELL15_INCDIR], $litecell15_incdir)
 AC_MSG_RESULT([$enable_lc15bts_phy])
 AM_CONDITIONAL(ENABLE_LC15BTS_PHY, test "x$enable_lc15bts_phy" = "xyes")