libosmogb: make build conditional, disable in embedded build

when we build for osmocom-bb cross-compilation, we don't have
sockets nor netinet/in.h and thus NS/BSSGP make no sense...
diff --git a/configure.ac b/configure.ac
index ca98fc2..f119c90 100644
--- a/configure.ac
+++ b/configure.ac
@@ -137,6 +137,14 @@
         [enable_utilities=$enableval], [enable_utilities="yes"])
 AM_CONDITIONAL(ENABLE_UTILITIES, test x"$enable_utilities" = x"yes")
 
+AC_ARG_ENABLE(gb,
+        [AS_HELP_STRING(
+                [--disable-gb],
+                [Disable building Gb library],
+        )],
+        [enable_gb=$enableval], [enable_gb="yes"])
+AM_CONDITIONAL(ENABLE_GB, test x"$enable_gb" = x"yes")
+
 AC_ARG_ENABLE(embedded,
 	[AS_HELP_STRING(
 		[--enable-embedded],
@@ -152,6 +160,7 @@
 	AM_CONDITIONAL(ENABLE_VTY, false)
 	AM_CONDITIONAL(ENABLE_TALLOC, false)
 	AM_CONDITIONAL(ENABLE_UTILITIES, false)
+	AM_CONDITIONAL(ENABLE_GB, false)
 	AC_DEFINE([PANIC_INFLOOP],[1],[Use infinite loop on panic rather than fprintf/abort])
 fi