build: a few fixes to configure script

[bluetech]

• Use C99 and gnu extensions.
• Explicitly link against libdrm (more strict linkers like gold would complain).
• Consistently use *_CPPFLAGS where due.
• Clear the default CFLAGS.

Signed-off-by: Ran Benita ran234@gmail.com

[dvdhrm]

Pulled, thanks.

Cheers David