add the compiler option --enable-debugging

configure.ac

@@ -82,6 +82,24 @@ if test "x$enable_stack_protector" != "xno"; then
 		[], [])
 fi
 
+# --enable-debugging
+AC_ARG_ENABLE(debugging,
+	AS_HELP_STRING([--enable-debugging],[enable debugging (default is NO)]),
+	[],
+	[enable_debugging=no])
+
+# AC_PROG_CC sets CFLAGS to '-g -O2' unless it is already set, so
+# there's no need to add -g. Disable or change by explicitly setting
+# CFLAGS. If this option is enabled, then -Og or -O0 becomes the last
+# optimization option, and hence takes precedence.
+if test "x$enable_debugging" != "xno"; then
+	CFLAGS="${CFLAGS} -fno-inline"
+	AX_CHECK_COMPILE_FLAG([-Og],
+		[CFLAGS="${CFLAGS} -Og"],
+		[CFLAGS="${CFLAGS} -O0"],
+		[])
+fi
+
 AC_CONFIG_FILES([
 	Makefile
 	src/Makefile

src/vmod_blobcode.vcc

@@ -290,6 +290,23 @@ Other files such as this man-page are installed in the locations
 determined by ``configure``, which inherits its default ``--prefix``
 setting from Varnish.
 
+For developers
+--------------
+
+The build specifies C99 conformance, all compiler warnings turned on,
+and all warnings considered errors (compiler options ``-std=c99
+-Werror -Wall``).
+
+By default, ``CFLAGS`` is set to ``-g -O2``, so that symbols are
+included in the shared library, and optimization is at level
+``O2``. To change or disable these options, set ``CFLAGS`` explicitly
+before calling ``make`` (it may be set to the empty string).
+
+For development/debugging cycles, the ``configure`` option
+``--enable-debugging`` is recommended (off by default). This will turn
+off optimizations and function inlining, so that a debugger will step
+through the code as expected.
+
 LIMITATIONS
 ===========
 
@@ -308,7 +325,7 @@ temporary BLOB. If this function causes stack overflow, you may need
 to increase the stack size for the varnishd process, for example with
 ``ulimit -s``. By default, the VMOD is built with the stack protector
 enabled (compile option ``-fstack-protector``), but it can be disabled
-with the ``./configure`` option ``--disable-stack-protector``.
+with the ``configure`` option ``--disable-stack-protector``.
 
 AUTHORS
 =======