Remove `LIBC_INSTALL_PREFIX`

[Ericson2314]

Summary

This matches the decision made in D99697.

It also shouldn't reintroduce the issue fixed in D99636.

The variable was originally introduced in
b22f448c21e718a3b6219df89169f38d436189c6 but is not essential to that
change.

Once we finish adding GnuInstallDirs support in D100810 and D99484,
setting CMAKE_INSTALL_LIBDIR would also work to change the
installation directory (though for more than libc).

GnuInstallDirs support also brings up an issue which is avoided if
variables like LIBC_INSTALL_PREFIX don't exist. Because the
GnuInstallDirs variables can be absolute paths, it is a bit unclear
how the per-project prefixes would work: does the project-agnostic
role-specific variable (e.g. CMAKE_INSTALL_LIBDIR), or project-specfic
role-agnostic (e.g. LIBC_INSTALL_PREFIX) take priority? Each is more
specific than the other on one axis, but not the other.

[Ericson2314]

@phosek also mind reviewing this one? An alternative would be the fancy concatenation logic I just did for compiler-rt, but I think that's a bit overkill when there is currently just one variable to be prepended.

[phosek]

LGTM

[Ericson2314]

Thanks!

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 ${CMAKE_INSTALL_PREFIX} CACHE PATH "Define libc destination prefix.")
-
 set(LIBC_TARGET_OS ${CMAKE_SYSTEM_NAME})
 string(TOLOWER ${LIBC_TARGET_OS} LIBC_TARGET_OS)
 
 # Defines LIBC_TARGET_ARCHITECTURE and associated macros.
 include(LLVMLibCArchitectures)
 
 # Flags to pass down to the compiler while building the libc functions.
 set(LIBC_COMPILE_OPTIONS_DEFAULT "" CACHE STRING "Architecture to tell clang to optimize for (e.g. -march=... or -mcpu=...)")

libc/lib/CMakeLists.txt

 add_entrypoint_library(
   llvmlibc
   DEPENDS
   ${TARGET_LLVMLIBC_ENTRYPOINTS}
 )
 
 if(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR)
   set(LIBC_INSTALL_LIBRARY_DIR lib${LLVM_LIBDIR_SUFFIX}/${LLVM_DEFAULT_TARGET_TRIPLE})
 else()
   set(LIBC_INSTALL_LIBRARY_DIR lib${LLVM_LIBDIR_SUFFIX})
 endif()
 
 install(
   TARGETS llvmlibc
-  ARCHIVE DESTINATION ${LIBC_INSTALL_PREFIX}/${LIBC_INSTALL_LIBRARY_DIR}
+  ARCHIVE DESTINATION "${LIBC_INSTALL_LIBRARY_DIR}"
   COMPONENT llvmlibc
 )
 
 add_custom_target(
     install-llvmlibc
     DEPENDS llvmlibc
     COMMAND "${CMAKE_COMMAND}" -DCMAKE_INSTALL_COMPONENT=llvmlibc
             -P "${LIBC_BINARY_DIR}/cmake_install.cmake"
 )