cmake: Make SECP256K1_INSTALL default depend on PROJECT_IS_TOP_LEVEL

Also full stops have been added to the option help texts for consistency
in cmake-gui.

Author: hebasto

CMakeLists.txt

@@ -38,13 +38,13 @@ set(CMAKE_C_EXTENSIONS OFF)
 
 list(APPEND CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/cmake)
 
-option(BUILD_SHARED_LIBS "Build shared libraries" ON)
+option(BUILD_SHARED_LIBS "Build shared libraries." ON)
 option(SECP256K1_DISABLE_SHARED "Disable shared library. Overrides BUILD_SHARED_LIBS." OFF)
 if(SECP256K1_DISABLE_SHARED)
   set(BUILD_SHARED_LIBS OFF)
 endif()
 
-option(SECP256K1_INSTALL "Enable installation" ON)
+option(SECP256K1_INSTALL "Enable installation." ${PROJECT_IS_TOP_LEVEL})
 
 option(SECP256K1_ENABLE_MODULE_ECDH "Enable ECDH module." ON)
 if(SECP256K1_ENABLE_MODULE_ECDH)