cmake: use install directories provided via GNUInstallDirs

We currently hand-code logic to configure where to install our artifacts
via the `LIB_INSTALL_DIR`, `INCLUDE_INSTALL_DIR` and `BIN_INSTALL_DIR`
variables. This is reinventing the wheel, as CMake already provide a way
to do that via `CMAKE_INSTALL_<DIR>` paths, e.g. `CMAKE_INSTALL_LIB`.
This requires users of libgit2 to know about the discrepancy and will
require special hacks for any build systems that handle these variables
in an automated way. One such example is Gentoo Linux, which sets up
these paths in both the cmake and cmake-utils eclass.

So let's stop doing that: the GNUInstallDirs module handles it in a
better way for us, especially so as the actual values are dependent on
CMAKE_INSTALL_PREFIX. This commit removes our own set of variables and
instead refers users to use the standard ones.

As a second benefit, this commit also fixes our pkgconfig generation to
use the GNUInstallDirs module. We had a bug there where we ignored the
CMAKE_INSTALL_PREFIX when configuring the libdir and includedir keys, so
if libdir was set to "lib64", then libdir would be an invalid path. With
GNUInstallDirs, we can now use `CMAKE_INSTALL_FULL_LIBDIR`, which
handles the prefix for us.

4 files changed, 12 insertions, 19 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 113e554d2..826415893 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -36,6 +36,7 @@ INCLUDE(AddCFlagIfSupported)
 INCLUDE(FindPkgLibraries)
 INCLUDE(FindThreads)
 INCLUDE(FindStatNsec)
+INCLUDE(GNUInstallDirs)
 INCLUDE(IdeSplitSources)
 INCLUDE(FeatureSummary)
 INCLUDE(EnableWarnings)
diff --git a/README.md b/README.md
index ef85fd9f6..b5f04403b 100644
--- a/README.md
+++ b/README.md
@@ -247,9 +247,9 @@ For more advanced use or questions about CMake please read <https://cmake.org/Wi
 
 The following CMake variables are declared:
 
-- `BIN_INSTALL_DIR`: Where to install binaries to.
-- `LIB_INSTALL_DIR`: Where to install libraries to.
-- `INCLUDE_INSTALL_DIR`: Where to install headers to.
+- `CMAKE_INSTALL_BINDIR`: Where to install binaries to.
+- `CMAKE_INSTALL_LIBDIR`: Where to install libraries to.
+- `CMAKE_INSTALL_INCLUDEDIR`: Where to install headers to.
 - `BUILD_SHARED_LIBS`: Build libgit2 as a Shared Library (defaults to ON)
 - `BUILD_CLAR`: Build [Clar](https://github.com/vmg/clar)-based test suite (defaults to ON)
 - `THREADSAFE`: Build libgit2 with threading support (defaults to ON)
diff --git a/cmake/Modules/PkgBuildConfig.cmake b/cmake/Modules/PkgBuildConfig.cmake
index e4c574431..54c5e294c 100644
--- a/cmake/Modules/PkgBuildConfig.cmake
+++ b/cmake/Modules/PkgBuildConfig.cmake
@@ -27,8 +27,8 @@ function(pkg_build_config)
     # Write .pc "header"
     file(WRITE "${PKGCONFIG_FILE}"
         "prefix=\"${CMAKE_INSTALL_PREFIX}\"\n"
-        "libdir=\"${LIB_INSTALL_DIR}\"\n"
-        "includedir=\"${INCLUDE_INSTALL_DIR}\"\n"
+        "libdir=\"${CMAKE_INSTALL_FULL_LIBDIR}\"\n"
+        "includedir=\"${CMAKE_INSTALL_FULL_INCLUDEDIR}\"\n"
         "\n"
         "Name: ${PKGCONFIG_NAME}\n"
         "Description: ${PKGCONFIG_DESCRIPTION}\n"
@@ -73,7 +73,5 @@ function(pkg_build_config)
     file(APPEND "${PKGCONFIG_FILE}" "Cflags: -I\${includedir} ${PKGCONFIG_CFLAGS}\n")
 
     # Install .pc file
-    install(FILES "${PKGCONFIG_FILE}"
-        DESTINATION "${LIB_INSTALL_DIR}/pkgconfig"
-    )
+    install(FILES "${PKGCONFIG_FILE}" DESTINATION "${CMAKE_INSTALL_LIBDIR}/pkgconfig")
 endfunction()
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 1915e8a9a..7e4738167 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -21,12 +21,6 @@ SET(LIBGIT2_INCLUDES
 SET(LIBGIT2_SYSTEM_INCLUDES "")
 SET(LIBGIT2_LIBS "")
 
-# Installation paths
-#
-SET(BIN_INSTALL_DIR bin CACHE PATH "Where to install binaries to.")
-SET(LIB_INSTALL_DIR lib CACHE PATH "Where to install libraries to.")
-SET(INCLUDE_INSTALL_DIR include CACHE PATH "Where to install headers to.")
-
 # Enable tracing
 IF(ENABLE_TRACE)
 	SET(GIT_TRACE 1)
@@ -387,9 +381,9 @@ ENDIF ()
 
 # Install
 INSTALL(TARGETS git2
-	RUNTIME DESTINATION ${BIN_INSTALL_DIR}
-	LIBRARY DESTINATION ${LIB_INSTALL_DIR}
-	ARCHIVE DESTINATION ${LIB_INSTALL_DIR}
+	RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
+	LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
+	ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
 )
-INSTALL(DIRECTORY ${libgit2_SOURCE_DIR}/include/git2 DESTINATION ${INCLUDE_INSTALL_DIR} )
-INSTALL(FILES ${libgit2_SOURCE_DIR}/include/git2.h DESTINATION ${INCLUDE_INSTALL_DIR} )
+INSTALL(DIRECTORY ${libgit2_SOURCE_DIR}/include/git2 DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
+INSTALL(FILES ${libgit2_SOURCE_DIR}/include/git2.h DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})