cmake: Simplify ccache logic

* Use USE_CCACHE switch, this seems to be a more common option having a quick look using Google
* Make use of find_program() functionality introduced in CMake 3.18 to simplify code

CMakeLists.txt

@@ -4,7 +4,7 @@
 # integer (MPI) library written entirely in C.
 #
 
-cmake_minimum_required(VERSION 3.10)
+cmake_minimum_required(VERSION 3.18)
 
 project(libtommath
     VERSION 1.3.0
@@ -36,15 +36,11 @@ option(BUILD_SHARED_LIBS "Build shared library and only the shared library if \"
 #-----------------------------------------------------------------------------
 # Add support for ccache if desired
 #-----------------------------------------------------------------------------
-find_program(CCACHE ccache)
+option(USE_CCACHE "Enable ccache" OFF)
 
-if(CCACHE)
-    option(ENABLE_CCACHE "Enable ccache." ON)
-endif()
-
-# use ccache if installed
-if(CCACHE AND ENABLE_CCACHE)
-    set(CMAKE_C_COMPILER_LAUNCHER ${CCACHE})
+if(USE_CCACHE)
+   find_program(CCACHE ccache REQUIRED)
+   set(CMAKE_C_COMPILER_LAUNCHER ${CCACHE})
 endif()
 
 #-----------------------------------------------------------------------------