Add build switch to enable auto ccache

Make usage of ccache by build system opt-in.

CMakeLists.txt

@@ -46,6 +46,7 @@ option( EXIV2_BUILD_SAMPLES           "Build sample applications"
 option( EXIV2_BUILD_PO                "Build translations files"                              OFF )
 option( EXIV2_BUILD_EXIV2_COMMAND     "Build exiv2 command-line executable"                   ON  )
 option( EXIV2_BUILD_UNIT_TESTS        "Build unit tests"                                      OFF )
+option( BUILD_WITH_CCACHE             "Use ccache to speed up compile time"                   OFF )
 
 if ( EXIV2_ENABLE_WEBREADY )
     set ( EXIV2_ENABLE_CURL ON )

config/findDependencies.cmake

@@ -58,11 +58,13 @@ if( EXIV2_BUILD_PO )
     endif()
 endif()
 
-find_program(CCACHE_FOUND ccache)
-if(CCACHE_FOUND)
-    message(STATUS "Program ccache found")
-    set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ccache)
-    set_property(GLOBAL PROPERTY RULE_LAUNCH_LINK ccache)
+if( BUILD_WITH_CCACHE )
+    find_program(CCACHE_FOUND ccache)
+    if(CCACHE_FOUND)
+        message(STATUS "Program ccache found")
+        set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ccache)
+        set_property(GLOBAL PROPERTY RULE_LAUNCH_LINK ccache)
+    endif()
 endif()
 
 if (EXIV2_BUILD_UNIT_TESTS)

config/printSummary.cmake

@@ -41,6 +41,7 @@ OptionOutput( "Building exiv2 command:             " EXIV2_BUILD_EXIV2_COMMAND
 OptionOutput( "Building samples:                   " EXIV2_BUILD_SAMPLES             )
 OptionOutput( "Building PO files:                  " EXIV2_BUILD_PO                  )
 OptionOutput( "Building unit tests:                " EXIV2_BUILD_UNIT_TESTS          )
+OptionOutput( "Using ccache:                       " BUILD_WITH_CCACHE               )
 
 
 message( STATUS "------------------------------------------------------------------" )