Use Imported Target for libgit2

CMake's pkg-config module does not add -L flags to xxx_LIBRARIES var, and so linking may fail if libgit2 isn't in a path known to the linker.

Use IMPORTED_TARGET argument in pkg_check_modules (available since CMake 3.6) to create a full imported target to libgit2. Update build and config file use to link to imported target.

cmake/GitCondDBConfig.cmake.in

@@ -12,10 +12,10 @@
 @PACKAGE_INIT@
 
 find_package(PkgConfig)
-pkg_check_modules(git2 libgit2 REQUIRED)
+pkg_check_modules(git2 libgit2 REQUIRED IMPORTED_TARGET)
 
 include("${CMAKE_CURRENT_LIST_DIR}/GitCondDBTargets.cmake")
 
 set_target_properties(GitCondDB PROPERTIES
-  INTERFACE_LINK_LIBRARIES "${git2_LIBRARIES}"
+  INTERFACE_LINK_LIBRARIES PkgConfig::git2
 )