cmake: fix compatibility with cmake before 3.3

The COMPILE_LANGUAGE generator expression is only supported since CMake
3.3. Moreover, it does not work with all generators (works with Makefile
and Ninja, but not with Visual Studio).

target_compile_options would only work if a target does not mix C and
C++ sources, since the flags are intended to be set for a specific
language, use set_source_files_properties instead. This approach is also
less repetitive.

Drop the idea of using lists and COMPILE_OPTIONS,
set_source_files_properties only understands COMPILE_FLAGS (a single
string, not a list).

examples/CMakeLists.txt

 if(ENABLE_EXAMPLES)
-  # XXX replace this by lists (WARNCFLAGS_list)
-  string(REPLACE " " ";" c_flags "${WARNCFLAGS}")
-  string(REPLACE " " ";" cxx_flags "${WARNCXXFLAGS} ${CXX1XCXXFLAGS}")
-  add_compile_options(
-    "$<$<COMPILE_LANGUAGE:C>:${c_flags}>"
-    "$<$<COMPILE_LANGUAGE:CXX>:${cxx_flags}>"
-  )
+  file(GLOB c_sources *.c)
+  set_source_files_properties(${c_sources} PROPERTIES
+    COMPILE_FLAGS "${WARNCFLAGS}")
+  file(GLOB cxx_sources *.cc)
+  set_source_files_properties(${cxx_sources} PROPERTIES
+    COMPILE_FLAGS "${WARNCXXFLAGS} ${CXX1XCXXFLAGS}")
 
   include_directories(
     ${CMAKE_SOURCE_DIR}

lib/CMakeLists.txt

@@ -26,7 +26,6 @@ set(NGHTTP2_SOURCES
 
 # Public shared library
 add_library(nghttp2 SHARED ${NGHTTP2_SOURCES})
-# Needed because the object files are linked into a shared library.
 set_target_properties(nghttp2 PROPERTIES
   COMPILE_FLAGS "${WARNCFLAGS}"
   VERSION ${LT_VERSION} SOVERSION ${LT_SOVERSION}

src/CMakeLists.txt

 add_subdirectory(includes)
 
-# XXX replace this by lists (WARNCFLAGS_list)
-string(REPLACE " " ";" c_flags "${WARNCFLAGS}")
-string(REPLACE " " ";" cxx_flags "${WARNCXXFLAGS} ${CXX1XCXXFLAGS}")
-add_compile_options(
-  "$<$<COMPILE_LANGUAGE:C>:${c_flags}>"
-  "$<$<COMPILE_LANGUAGE:CXX>:${cxx_flags}>"
-)
+file(GLOB c_sources *.c)
+set_source_files_properties(${c_sources} PROPERTIES
+  COMPILE_FLAGS "${WARNCFLAGS}")
+file(GLOB cxx_sources *.cc)
+set_source_files_properties(${cxx_sources} PROPERTIES
+  COMPILE_FLAGS "${WARNCXXFLAGS} ${CXX1XCXXFLAGS}")
 
 include_directories(
   "${CMAKE_SOURCE_DIR}/lib/includes"
@@ -246,7 +245,6 @@ if(ENABLE_ASIO_LIB)
     ${OPENSSL_LIBRARIES}
     ${Boost_LIBRARIES}
   )
-  # XXX use add_compile_options with a list instead of COMPILE_FLAGS
   set_target_properties(nghttp2_asio PROPERTIES
     VERSION 1.0.0 SOVERSION 1)
 

tests/CMakeLists.txt

 # XXX testdata/: EXTRA_DIST = cacert.pem  index.html  privkey.pem
 if(HAVE_CUNIT)
-  # XXX replace this by lists (WARNCFLAGS_list)
   string(REPLACE " " ";" c_flags "${WARNCFLAGS}")
   add_compile_options(${c_flags})