disable install targets when osm2pgsql is inside a subdir

When osm2pgsql is included into another project as a subproject
then it should be the main project which decides what gets
installed.

This is mainly useful for nominatim which needs to install its
own copy of osm2pgsql, potentially alongside an existing
stand-alone installation.

Author: lonvia

CMakeLists.txt

@@ -10,6 +10,15 @@ set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
 
+# Do not create install targets when run as a subproject.
+# Currently used by Nominatim which cannot yet rely on installed versions
+# of osm2pgsql.
+if (${PROJECT_NAME} STREQUAL ${CMAKE_PROJECT_NAME})
+    set(ENABLE_INSTALL ON)
+else()
+    set(ENABLE_INSTALL OFF)
+endif()
+
 if (WIN32)
     set(DEFAULT_STYLE "default.style" CACHE STRING "Default style used unless one is given on the command line")
 else()
@@ -324,5 +333,7 @@ add_subdirectory(docs)
 # Install
 #############################################################
 
-install(TARGETS osm2pgsql DESTINATION bin)
-install(FILES default.style empty.style DESTINATION share/osm2pgsql)
+if (ENABLE_INSTALL)
+    install(TARGETS osm2pgsql DESTINATION bin)
+    install(FILES default.style empty.style DESTINATION share/osm2pgsql)
+endif()

docs/CMakeLists.txt

@@ -27,5 +27,6 @@ else()
     message(STATUS "  Manual page can not be built")
 endif()
 
-install(FILES osm2pgsql.1 DESTINATION share/man/man1)
-
+if (ENABLE_INSTALL)
+    install(FILES osm2pgsql.1 DESTINATION share/man/man1)
+endif()