Fix installed target definition.

Using CMAKE_INSTALL_INCLUDEDIR before including GNUINstallDirs results
in a broken installation when CMAKE_INSTALL_PREFIX is a non-standard
directory.

Inspired from google/crc32c#39

PiperOrigin-RevId: 278427974

Author: pwnall

CMakeLists.txt

@@ -106,6 +106,9 @@ if(BUILD_SHARED_LIBS)
   add_compile_options(-fvisibility=hidden)
 endif(BUILD_SHARED_LIBS)
 
+# Must be included before CMAKE_INSTALL_INCLUDEDIR is used.
+include(GNUInstallDirs)
+
 add_library(leveldb "")
 target_sources(leveldb
   PRIVATE
@@ -417,7 +420,6 @@ int main() {
 endif(LEVELDB_BUILD_BENCHMARKS)
 
 if(LEVELDB_INSTALL)
-  include(GNUInstallDirs)
   install(TARGETS leveldb
     EXPORT leveldbTargets
     RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}