CMake: add BUILD_SHARED_LIBS option

Prior to commit 4e955731, ngtcp2 was
built as shared library. That has changed into a static or shared
library depending on the BUILD_SHARED_LIBS option, but this option was
not documented and the comments were misleading. Rectify both issues.

Note that autotools builds both libraries by default, but the current
CMake config is only able to build one of them.

CMakeLists.txt

@@ -313,6 +313,8 @@ message(STATUS "summary of build options:
       CXXFLAGS:       ${CMAKE_CXX_FLAGS_${_build_type}} ${CMAKE_CXX_FLAGS}
       WARNCFLAGS:     ${WARNCFLAGS}
       WARNCXXFLAGS:   ${WARNCXXFLAGS}
+    Library:
+      Shared:         ${BUILD_SHARED_LIBS}
     Test:
       CUnit:          ${HAVE_CUNIT} (LIBS='${CUNIT_LIBRARIES}')
     Libs:

CMakeOptions.txt

 # Features that can be enabled for cmake (see CMakeLists.txt)
 
+option(BUILD_SHARED_LIBS "Build using shared libraries" OFF)
+
 option(ENABLE_WERROR    "Make compiler warnings fatal" OFF)
 option(ENABLE_DEBUG     "Turn on debug output" OFF)
 option(ENABLE_ASAN      "Enable AddressSanitizer (ASAN)" OFF)

crypto/gnutls/CMakeLists.txt

@@ -41,7 +41,7 @@ foreach(name libngtcp2_crypto_gnutls.pc)
   configure_file("${name}.in" "${name}" @ONLY)
 endforeach()
 
-# Public shared library
+# Public shared or static library
 add_library(ngtcp2_crypto_gnutls ${ngtcp2_crypto_gnutls_SOURCES})
 set_target_properties(ngtcp2_crypto_gnutls PROPERTIES
   COMPILE_FLAGS "${WARNCFLAGS}"

crypto/openssl/CMakeLists.txt

@@ -41,7 +41,7 @@ foreach(name libngtcp2_crypto_openssl.pc)
   configure_file("${name}.in" "${name}" @ONLY)
 endforeach()
 
-# Public shared library
+# Public shared or static library
 add_library(ngtcp2_crypto_openssl ${ngtcp2_crypto_openssl_SOURCES})
 set_target_properties(ngtcp2_crypto_openssl PROPERTIES
   COMPILE_FLAGS "${WARNCFLAGS}"

lib/CMakeLists.txt

@@ -64,7 +64,7 @@ set(ngtcp2_INCLUDE_DIRS
   "${CMAKE_CURRENT_BINARY_DIR}/includes"
 )
 
-# Public shared library
+# Public shared or static library
 add_library(ngtcp2 ${ngtcp2_SOURCES})
 set_target_properties(ngtcp2 PROPERTIES
   COMPILE_FLAGS "${WARNCFLAGS}"
@@ -79,7 +79,7 @@ if(NOT BUILD_SHARED_LIBS)
 endif()
 
 if(HAVE_CUNIT)
-  # Static library (for unittests because of symbol visibility)
+  # Private static library (for unittests because of symbol visibility)
   add_library(ngtcp2_static STATIC ${ngtcp2_SOURCES})
   set_target_properties(ngtcp2_static PROPERTIES
     COMPILE_FLAGS "${WARNCFLAGS}"