Clang: Add -Wno-error=undefined-func-template

Recent clang versions include this diagnostic.  Until it is confirmed
whether this is a false positive, a library misuse or an upstream bug,
it should stay in as a warning.

cmake/compiler_clang.cmake

@@ -35,6 +35,25 @@ add_definitions(-Wno-error=undefined-reinterpret-cast)
 # Clang on recent XCode fails to compile the boost tests
 add_definitions(-Wno-error=disabled-macro-expansion)
 
+# Clang-5.0 notices this on runtimeconfiguration.cpp:201:14
+#
+# 'boost::program_options::parse_config_file<char>' required here, but no
+# definition is available [-Werror,-Wundefined-func-template]
+# store( parse_config_file( cfile, configFileOptions), vm);
+#
+# FIXME: This is a workaround, not a solution.
+# See #191 at https://github.com/dannyedel/dspdfviewer/issues/191
+#
+# Older clang versions (at least including clang-3.8) abort compilation
+# with this flag, so first check if this current version supports it.
+Include(CheckCXXCompilerFlag)
+CHECK_CXX_COMPILER_FLAG("-Wno-error=undefined-func-template"
+	CLANG_SUPPORTS_UNDEFINED_FUNC_TEMPLATE_DIAGNOSTIC)
+if(CLANG_SUPPORTS_UNDEFINED_FUNC_TEMPLATE_DIAGNOSTIC)
+	# Add the command-l
+	add_definitions(-Wno-error=undefined-func-template)
+endif()
+
 #message(FATAL_ERROR "Version: ${CMAKE_CXX_COMPILER_VERSION}")
 
 # clang will complain about -isystem passed but not used.