build: Use AC_CACHE_SAVE

Delimit the points in the configure script that should save the state,
so that running the script multiple times doesn't require starting from
scratch even if it didn't terminate successfully.

configure.ac

@@ -83,6 +83,8 @@ AC_SUBST([CLUTTER_LT_VERSION],  [$CLUTTER_LT_VERSION])
 
 AC_PROG_SED
 
+dnl = Preliminary platform checks =============================================
+
 AC_CANONICAL_HOST
 
 platform_win32=no
@@ -112,7 +114,9 @@ AM_CONDITIONAL(OS_GLX, [test "$platform_glx" = "yes"])
 
 AC_SUBST(CLUTTER_LT_LDFLAGS)
 
-dnl ========================================================================
+AC_CACHE_SAVE
+
+dnl = Dependencies =========================================================
 
 # Checks for programs.
 AM_PROG_CC_C_O
@@ -181,6 +185,8 @@ AC_ARG_ENABLE([Bsymbolic],
 AS_IF([test "x$enable_Bsymbolic" = "xyes"], [CLUTTER_LINK_FLAGS=-Wl[,]-Bsymbolic-functions])
 AC_SUBST(CLUTTER_LINK_FLAGS)
 
+AC_CACHE_SAVE
+
 dnl ========================================================================
 
 FLAVOUR_LIBS=""
@@ -453,6 +459,8 @@ dnl strip leading spaces
 CLUTTER_BACKENDS=${CLUTTER_BACKENDS#* }
 AC_SUBST(CLUTTER_BACKENDS)
 
+AC_CACHE_SAVE
+
 dnl === Clutter configuration =================================================
 
 CLUTTER_CONFIG_DEFINES=