plug-ins: fix the build on OS X

We need to have -xobjective-c specified only for compile commands
that will perform compilation. Otherwise, for compile commands
that only link the compiler interprets .o files as C source ...

plug-ins/common/Makefile.am

@@ -1769,7 +1769,7 @@ warp_LDADD = \
 
 web_browser_LDFLAGS = $(framework_cocoa)
 
-web_browser_CFLAGS = $(xobjective_c)
+web_browser_CPPFLAGS = $(AM_CPPFLAGS) $(xobjective_c)
 
 web_browser_SOURCES = \
 	web-browser.c