Makes the port build with clang when the ALSA option is on.

commit d69277128afb60999e420c533e826b0061fc40eb
Author: Raphael Kubo da Costa <rakuco@FreeBSD.org>
Date:   Sat Nov 28 14:27:45 2015 +0100

    Extend -std=c99 compiler flag to clang.

    The ALSA headers use features such as the `inline' keyword that require
    C99 support. Commit e36c628 ("Fix compilation: ALSA no longer compiles
    with -std=c90") started passing it to the compiler when GCC is used.

    We now do the same for clang with a better comment, and also append the
    flag to the existing CMAKE_C_FLAGS, otherwise we lose several other
    options which are set by FindKDE4Internal.cmake.

--- CMakeLists.txt
+++ CMakeLists.txt
@@ -7,9 +7,12 @@ include(MacroLibrary)
 add_definitions(${QT_DEFINITIONS} ${KDE4_DEFINITIONS})
 
 ## Compiler flags
-if(CMAKE_COMPILER_IS_GNUCXX)
-    set(CMAKE_C_FLAGS "-std=c99")   ## ALSA no longer compiles with -std=c90, see https://bugzilla.novell.com/show_bug.cgi?id=817077
+if (CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+    # ALSA does not build with the default -std=iso9899:1990 from FindKDE4Internal.cmake.
+    # See https://bugzilla.novell.com/show_bug.cgi?id=817077 for more information.
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c99")
 endif()
+
 find_package(Alsa)
 alsa_configure_file(${CMAKE_CURRENT_BINARY_DIR}/config-alsa.h)