CMake: Use system libgit2 if found

This makes it possible to use the GitHub source release as it doesn't
contain the submodule. Also avoids unnecessary build of libgit2.

Author: jhasse

CMakeLists.txt

@@ -5,7 +5,14 @@ SET(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
 SET(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
 
 # Add libgit2
-ADD_SUBDIRECTORY(ext/libgit2 ${CMAKE_BINARY_DIR}/lib)
+find_package(PkgConfig)
+if(PkgConfig_FOUND)
+  PKG_CHECK_MODULES(LIBGIT2 libgit2>=0.99)
+endif()
+if(NOT LIBGIT2_FOUND)
+  ADD_SUBDIRECTORY(ext/libgit2 ${CMAKE_BINARY_DIR}/lib)
+  set(LIBGIT2_INCLUDEDIR ext/libgit2/include)
+endif()
 
 INCLUDE(CMakePackageConfigHelpers)
 
@@ -20,7 +27,7 @@ SET(CPPGIT2_VERSION_STRING "${CPPGIT2_VERSION_MAJOR}.${CPPGIT2_VERSION_MINOR}.${
 
 # Build object library
 ADD_LIBRARY(CPPGIT2_OBJECT_LIBRARY OBJECT ${CPPGIT2_SOURCES})
-INCLUDE_DIRECTORIES("include" "ext/libgit2/include" "test")
+INCLUDE_DIRECTORIES("include" "${LIBGIT2_INCLUDEDIR}" "test")
 SET_PROPERTY(TARGET CPPGIT2_OBJECT_LIBRARY PROPERTY CXX_STANDARD 11)
 
 # Shared libraries need PIC
@@ -42,7 +49,8 @@ TARGET_LINK_LIBRARIES(CPPGIT2_STATIC git2)
 
 # Copy include directories to build/include
 FILE(COPY "include" DESTINATION "${CMAKE_BINARY_DIR}/.")
-FILE(COPY "ext/libgit2/include" DESTINATION "${CMAKE_BINARY_DIR}/.")
+FILE(COPY "${LIBGIT2_INCLUDEDIR}/git2" DESTINATION "${CMAKE_BINARY_DIR}/include")
+FILE(COPY "${LIBGIT2_INCLUDEDIR}/git2.h" DESTINATION "${CMAKE_BINARY_DIR}/include")
 
 # Build samples
 FILE(GLOB SAMPLES "samples/*.cpp")