Build and install the memenv static library and header.

--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -208,6 +208,16 @@ target_sources(leveldb
     $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
 )
 
+add_library(memenv STATIC
+  "${PROJECT_SOURCE_DIR}/helpers/memenv/memenv.cc"
+  "${PROJECT_SOURCE_DIR}/helpers/memenv/memenv.h"
+)
+target_include_directories(memenv
+  PUBLIC
+    $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>
+    $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
+)
+
 set_target_properties(leveldb
   PROPERTIES VERSION ${PROJECT_VERSION} SOVERSION ${PROJECT_VERSION_MAJOR}d)
 
@@ -231,6 +249,12 @@ if (NOT HAVE_CXX17_HAS_INCLUDE)
   )
 endif(NOT HAVE_CXX17_HAS_INCLUDE)
 
+target_compile_definitions(memenv
+  PRIVATE
+    LEVELDB_COMPILE_LIBRARY
+    ${LEVELDB_PLATFORM_NAME}=1
+)
+
 if(BUILD_SHARED_LIBS)
   target_compile_definitions(leveldb
     PUBLIC
@@ -448,4 +476,9 @@ if(LEVELDB_INSTALL)
       "${PROJECT_BINARY_DIR}/leveldbConfigVersion.cmake"
     DESTINATION "${CMAKE_INSTALL_LIBDIR}/cmake/leveldb"
   )
+  install(
+    FILES "${PROJECT_SOURCE_DIR}/helpers/memenv/memenv.h"
+    DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/leveldb/helpers
+  )
+  install(TARGETS memenv ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR})
 endif(LEVELDB_INSTALL)