Add ccache switch, useful for local rebuilds

kernel.spec

@@ -85,6 +85,8 @@
 %bcond_with binary_extra_modules
 %endif
 
+%bcond_with ccache
+
 # Kernel flavour
 %if %{with nickel}
 %define flavour		nickel
@@ -175,11 +177,13 @@
 %define debug_package 		%{nil}
 %endif
 
-%if %(if [ -z "$CC" ] ; then echo 0; else echo 1; fi)
-%define kmake %make CC="$CC"
+# http://nickdesaulniers.github.io/blog/2018/06/02/speeding-up-linux-kernel-builds-with-ccache/
+%if %{with ccache}
+%define kmake KBUILD_BUILD_TIMESTAMP='' %make CC='ccache gcc'
 %else
-%define kmake %make
+%define kmake %make CC='gcc'
 %endif
+
 # there are places where parallel make don't work
 %define smake make
 
@@ -338,6 +342,10 @@ BuildRequires: kernel-source-virtualbox
 BuildRequires: kernel-source-virtualbox-vboxadditions
 %endif
 
+%if %{with ccache}
+BuildRequires: ccache
+%endif
+
 # might be useful too:
 Recommends:		microcode