build: include the flags for the build type in the _FORTIFY_SOURCE check

It turns out the check was being performed without optimizations enabled
even when the CMAKE_BUILD_TYPE was set to a release build.  This led to
_FORTIFY_SOURCE's level not being correctly determined, and us failing
to apply the correct workaround.

To counter this, we'll take the default flags for the build type and
apply them.  Also, if options are passed via CFLAGS, they are
automatically passed on to the underlying build.  So this should cover
all the necessary ground.

This fixes #1647.
Detect and propagate _FORTIFY_SOURCE prefix

Get prefix to a -D_FORTIFY_SOURCE string if it is present in
CFLAGS and apply the prefix to flags added to redefine
_FORTIFY_SOURCE in CFLAGS and CPPFLAGS

* fixes 1569
Merge pull request #1654 from gaurdro/rpm-fortify_source

[RFC] Fix issue 1569: Build fails in rpmbuild environment
Merge pull request #1686 from jszakmeister/include-opts-in-fortify-check

build: include the flags for the build type in the _FORTIFY_SOURCE check