Add a config note and fix a config variable regarding CCACHE support.

[mattd]

Summary

• Mention in the CMake configuration documentation that LLVM builds, support the use of ccache.
• Use CCACHE_MAXSIZE instead of CCACHE_SIZE. The former is recognized by ccache, not the latter.
• Rename the LLVM CMake configuration variable to reflect that change: LLVM_CCACHE_SIZE -> LLVM_CCACHE_MAXSIZE

[probinson]

I know off-list I had suggested not bothering with detailed descriptions of LLVM_CCACHE_SIZE and LLVM_CCACHE_DIR, but now I am not so sure. If those variables map directly to ccache command-line arguments, we should say that here, and people can figure out how to set them by looking at the appropriate help. If the variables are more complicated, we should add separate entries for them in this doc.

[probinson]

See inline comment. I think that makes it clear enough where to look for details about what values to use for the MAXSIZE/DIR parameters.

docs/CMake.rst

559

... options, which are passed to the CCACHE_MAXSIZE and CCACHE_DIR environment variables, respectively.

[mattd]

Thanks Paul, I like that suggestion.

[probinson]

LGTM

[mattd]

Ping. Since I don't have commit access I'll need someone else to commit this on my behalf.

[sgundapa]

Thanks for taking care of this

[mattd]

No problem!

CMakeLists.txt

 if(LLVM_CCACHE_BUILD)
   find_program(CCACHE_PROGRAM ccache)
   if(CCACHE_PROGRAM)
-      set(LLVM_CCACHE_SIZE "" CACHE STRING "Size of ccache")
+      set(LLVM_CCACHE_MAXSIZE "" CACHE STRING "Size of ccache")
       set(LLVM_CCACHE_DIR "" CACHE STRING "Directory to keep ccached data")
       set(CCACHE_PROGRAM "CCACHE_CPP2=yes CCACHE_HASHDIR=yes ${CCACHE_PROGRAM}")
-      if (LLVM_CCACHE_SIZE)
-        set(CCACHE_PROGRAM "CCACHE_SIZE=${LLVM_CCACHE_SIZE} ${CCACHE_PROGRAM}")
+      if (LLVM_CCACHE_MAXSIZE)
+        set(CCACHE_PROGRAM "CCACHE_MAXSIZE=${LLVM_CCACHE_MAXSIZE} ${CCACHE_PROGRAM}")
       endif()
       if (LLVM_CCACHE_DIR)
         set(CCACHE_PROGRAM "CCACHE_DIR=${LLVM_CCACHE_DIR} ${CCACHE_PROGRAM}")

docs/CMake.rst

   <http://clang.llvm.org/docs/SourceBasedCodeCoverage.html>`_ instrumentation
   is enabled while building llvm.
 
+**LLVM_CCACHE_BUILD**:BOOL
+  If enabled and the ``ccache`` program is available, then LLVM will be
+  built using ``ccache`` to speed up rebuilds of LLVM and its components.
+  Defaults to OFF.  The size and location of the cache maintained
+  by ``ccache`` can be adjusted via the LLVM_CCACHE_MAXSIZE and LLVM_CCACHE_DIR
+  options respectively.
+
 CMake Caches
 ============