libc: Default LIBC_INSTALL_PREFIX to ${CMAKE_INSTALL_PREFIX}

[nathanchance]

Summary

b22f448c21e718a3b6 added a rule to install libllvmlibc.a to
${LIBC_INSTALL_PREFIX}/${LIBC_INSTALL_LIBRARY_DIR}, which will be /lib
by default, which is disruptive to builds that stay within a user's
/home holder:

$ ninja install
...
-- Installing: /lib/libllvmlibc.a
CMake Error at projects/libc/lib/cmake_install.cmake:54 (file):
  file INSTALL cannot copy file
 "/home/nathan/cbl/github/tc-build/build/llvm/stage1/projects/libc/lib/libllvmlibc.a"
  to "/lib/libllvmlibc.a": Permission denied.
Call Stack (most recent call first):
  projects/libc/cmake_install.cmake:51 (include)
  projects/cmake_install.cmake:47 (include)
  cmake_install.cmake:76 (include)
...

Change LIBC_INSTALL_PREFIX's default value to ${CMAKE_INSTALL_PREFIX} so
that 'ninja install' does not attempt to install anything outside of the
user's requested installation location.

Signed-off-by: Nathan Chancellor <[e-mail]>

libc/CMakeLists.txt

 cmake_minimum_required(VERSION 3.13.4)
 
 # Use old version of target_sources command which converts the source
 # file paths to full paths.
 cmake_policy(SET CMP0076 OLD)
 list(APPEND CMAKE_MODULE_PATH  "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules")
 
 # The top-level sourse and binary directories.
 set(LIBC_SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR})
 set(LIBC_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR})
 # The top-level directory in which libc is being built.
 set(LIBC_BUILD_DIR ${CMAKE_CURRENT_BINARY_DIR})
 
 # Path libc/scripts directory.
 set(LIBC_BUILD_SCRIPTS_DIR "${LIBC_SOURCE_DIR}/utils/build_scripts")
 
-set(LIBC_INSTALL_PREFIX "" CACHE STRING "Define libc destination prefix.")
+set(LIBC_INSTALL_PREFIX ${CMAKE_INSTALL_PREFIX} CACHE PATH "Define libc destination prefix.")
 
 set(LIBC_TARGET_OS ${CMAKE_SYSTEM_NAME})
 string(TOLOWER ${LIBC_TARGET_OS} LIBC_TARGET_OS)
 
 set(LIBC_TARGET_MACHINE ${CMAKE_SYSTEM_PROCESSOR})
 
 # Check --print-resource-dir to find the compiler resource dir if this flag
 # is supported by the compiler.