cmake: option to disable install & drop curlu target when unused

This patch makes the following changes:
- adds the option `CURL_DISABLE_INSTALL` - to disable 'install' targets.
- Removes the target `curlu` when the option `BUILD_TESTING` is set to
  `OFF` - to prevent it from being loaded in Visual Studio.

Closes #12287

CMakeLists.txt

@@ -105,6 +105,8 @@ option(BUILD_SHARED_LIBS "Build shared libraries" ON)
 option(BUILD_STATIC_LIBS "Build static libraries" OFF)
 option(BUILD_STATIC_CURL "Build curl executable with static libcurl" OFF)
 option(ENABLE_ARES "Set to ON to enable c-ares support" OFF)
+option(CURL_DISABLE_INSTALL "Set to ON to disable instalation targets" OFF)
+
 if(WIN32)
   option(CURL_STATIC_CRT "Set to ON to build libcurl with static CRT on Windows (/MT)." OFF)
   option(ENABLE_UNICODE "Set to ON to use the Unicode version of the Windows API functions" OFF)
@@ -1523,6 +1525,8 @@ if(BUILD_TESTING)
   add_subdirectory(tests)
 endif()
 
+if(NOT CURL_DISABLE_INSTALL)
+
   # Helper to populate a list (_items) with a label when conditions (the remaining
   # args) are satisfied
   macro(_add_if label)
@@ -1779,3 +1783,4 @@ if(NOT TARGET curl_uninstall)
         COMMAND ${CMAKE_COMMAND} -P
         ${CMAKE_CURRENT_BINARY_DIR}/CMake/cmake_uninstall.cmake)
   endif()
+endif()

lib/CMakeLists.txt

@@ -47,6 +47,7 @@ if(USE_ARES)
   include_directories(${CARES_INCLUDE_DIR})
 endif()
 
+if(BUILD_TESTING)
   add_library(
     curlu # special libcurlu library just for unittests
     STATIC
@@ -54,13 +55,17 @@ add_library(
     ${HHEADERS} ${CSOURCES}
   )
   target_compile_definitions(curlu PUBLIC UNITTESTS CURL_STATICLIB)
+endif()
 
 if(ENABLE_CURLDEBUG)
   # We must compile these sources separately to avoid memdebug.h redefinitions
   # applying to them.
   set_source_files_properties(memdebug.c curl_multibyte.c PROPERTIES SKIP_UNITY_BUILD_INCLUSION ON)
 endif()
+
+if(BUILD_TESTING)
   target_link_libraries(curlu PRIVATE ${CURL_LIBS})
+endif()
 
 transform_makefile_inc("Makefile.soname" "${CMAKE_CURRENT_BINARY_DIR}/Makefile.soname.cmake")
 include(${CMAKE_CURRENT_BINARY_DIR}/Makefile.soname.cmake)