Only generate depends if the compiler supports it.

For assembly files, AS_DEPENDS needs to be set. As usually AS is set to
CC, it can be done using AS_DEPENDS = ${CC_DEPENDS}.

buildsys.mk.in

@@ -60,6 +60,10 @@ PLUGIN_SUFFIX = @PLUGIN_SUFFIX@
 INSTALL_LIB = @INSTALL_LIB@
 UNINSTALL_LIB = @UNINSTALL_LIB@
 CLEAN_LIB = @CLEAN_LIB@
+CC_DEPENDS = @CC_DEPENDS@
+CXX_DEPENDS = @CXX_DEPENDS@
+OBJC_DEPENDS = @OBJC_DEPENDS@
+OBJCXX_DEPENDS = @OBJCXX_DEPENDS@
 LN_S = @LN_S@
 MKDIR_P = mkdir -p
 INSTALL = @INSTALL@
@@ -126,9 +130,35 @@ depend: pre-depend ${SRCS}
 	test -f .deps || regen=1; \
 	for i in ${SRCS}; do \
 		case $$i in \
-			*.c | *.cc | *.cxx | *.m | *.mm | *.S) \
-				test $$i -nt .deps && regen=1; \
-				deps="$$deps $$i.dep"; \
+			*.c) \
+				if test x"${CC_DEPENDS}" = x"yes"; then \
+					test $$i -nt .deps && regen=1; \
+					deps="$$deps $$i.dep"; \
+				fi; \
+				;; \
+			*.cc | *.cxx) \
+				if test x"${CXX_DEPENDS}" = x"yes"; then \
+					test $$i -nt .deps && regen=1; \
+					deps="$$deps $$i.dep"; \
+				fi; \
+				;; \
+			*.m) \
+				if test x"${OBJC_DEPENDS}" = x"yes"; then \
+					test $$i -nt .deps && regen=1; \
+					deps="$$deps $$i.dep"; \
+				fi; \
+				;; \
+			*.mm) \
+				if test x"${OBJCXX_DEPENDS}" = x"yes"; then \
+					test $$i -nt .deps && regen=1; \
+					deps="$$deps $$i.dep"; \
+				fi; \
+				;; \
+			*.S) \
+				if test x"${AS_DEPENDS}" = x"yes"; then \
+					test $$i -nt .deps && regen=1; \
+					deps="$$deps $$i.dep"; \
+				fi; \
 				;; \
 		esac; \
 	done; \

m4/buildsys.m4

@@ -22,6 +22,11 @@ dnl POSSIBILITY OF SUCH DAMAGE.
 dnl
 
 AC_DEFUN([BUILDSYS_INIT], [
+	AC_SUBST(CC_DEPENDS, $GCC)
+	AC_SUBST(CXX_DEPENDS, $GXX)
+	AC_SUBST(OBJC_DEPENDS, $GOBJC)
+	AC_SUBST(OBJCXX_DEPENDS, $GOBJCXX)
+
 	AC_PATH_PROG(TPUT, tput)
 
 	AS_IF([test x"$TPUT" != x""], [