build: Set C dialect and warning level

build: Set C dialect and warning level

Currently, the Makefile is not setting the C dialect but the code was
carved out from the nvme-cli code base which relies on gnu99.
Let's define the C dialect to avoid any confusion and make the build
more consistent.

Without defining the default warning level the build system will use
the default level, which is implementation depended.  For example muon
sets it to 3 which includes '-Wpendantic'.  This results in a lot of
ISO-C non compliant warnings.

Let's define the warning level so that all build systems are using the
same values.

Signed-off-by: Daniel Wagner <dwagner@suse.de>