clang: Keep -qrtems and -B for compilers without -specs support

This is needed for building rtems correctly with Clang.

Note that this change does not mean rtems can build correctly with
mainline clang. However the change allows building rtems with a
Clang toolchain that has a rtems frontend similar that of GCC's
builtin rtems specs.

c/src/aclocal/prog-cc.m4

@@ -28,10 +28,12 @@ test "$rtems_cv_gcc_pipe" = "yes" && CC="$CC --pipe"
 dnl check if the compiler supports --specs
 RTEMS_GCC_SPECS
 
-AS_IF([test x"$rtems_cv_gcc_specs" = xyes],[
+dnl Compilers that does not support --specs (Clang) still needs -B and -qrtems
 GCCSPECS="-B\$(RTEMS_SOURCE_ROOT)/bsps/\$(RTEMS_CPU)/\$(RTEMS_BSP_FAMILY)/start"
-GCCSPECS="${GCCSPECS} -specs bsp_specs -qrtems"])
+AS_IF([test x"$rtems_cv_gcc_specs" = xyes],[
+GCCSPECS="${GCCSPECS} -specs bsp_specs"])
 AC_SUBST(GCCSPECS)
+GCCSPECS="${GCCSPECS} -qrtems"
 
 AS_IF([test "$GCC" = yes],[
   RTEMS_RELLDFLAGS="-qnolinkcmds -nostdlib -r"

testsuites/aclocal/gcc-specs.m4

@@ -0,0 +1,16 @@
+dnl Check whether the target compiler accepts -specs
+
+AC_DEFUN([RTEMS_GCC_SPECS],
+[AC_REQUIRE([RTEMS_PROG_CC])
+AC_CACHE_CHECK(whether $CC accepts -specs,rtems_cv_gcc_specs,
+[
+rtems_cv_gcc_specs=no
+if test x"$GCC" = x"yes"; then
+  touch confspec
+  echo 'void f(){}' >conftest.c
+  if test -z "`${CC} -specs confspec -c conftest.c 2>&1`";then
+    rtems_cv_gcc_specs=yes
+  fi
+fi
+rm -f confspec conftest*
+])])

testsuites/aclocal/prog-cc.m4

@@ -24,9 +24,14 @@ AC_DEFUN([RTEMS_PROG_CC_FOR_TARGET],
 dnl check target cc
 RTEMS_PROG_CC
 
+dnl check if the compiler supports -specs
+RTEMS_GCC_SPECS
+
 AS_IF([test x"$GCC" = xyes],[
 GCCSPECS="-B\$(RTEMS_SOURCE_ROOT)/bsps/\$(RTEMS_CPU)/\$(RTEMS_BSP_FAMILY)/start"
-GCCSPECS="${GCCSPECS} -specs bsp_specs -qrtems"])
+AS_IF([test x"$rtems_cv_gcc_specs" = xyes],[
+GCCSPECS="${GCCSPECS} -specs bsp_specs"])
+GCCSPECS="${GCCSPECS} -qrtems"])
 AC_SUBST(GCCSPECS)
 
 RTEMS_INCLUDES