Add subsurface-specific build setup

This changes the dc_version_suffix to show that this is the subsurface
'next generation' branch, and does minor tweaks to the build system: we
add the 'build' subdirectory to the .gitignore branch because that's
where we typically do our builds, and we tweak the default compiler
warning flags to not be as annoying.

No real semantic changes.

This is partially based off patches in the original Subsurface-branch by
Dirk Hohndel (configure.ac) and Jan Mulder (gitignore).  The sign-offs
for those come from those patches:

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Signed-off-by: Jan Mulder <jlmulder@xs4all.nl>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

.gitignore

@@ -82,3 +82,5 @@ Makefile.in
 /src/libdivecomputer.la
 /src/libdivecomputer.rc
 /src/revision.h
+
+/build

configure.ac

@@ -2,7 +2,7 @@
 m4_define([dc_version_major],[0])
 m4_define([dc_version_minor],[7])
 m4_define([dc_version_micro],[0])
-m4_define([dc_version_suffix],[devel])
+m4_define([dc_version_suffix],[devel-Subsurface-NG])
 m4_define([dc_version],dc_version_major.dc_version_minor.dc_version_micro[]m4_ifset([dc_version_suffix],-[dc_version_suffix]))
 
 # Libtool versioning.
@@ -175,19 +175,21 @@ AC_CHECK_FUNCS([getopt_long])
 
 # Checks for supported compiler options.
 AX_APPEND_COMPILE_FLAGS([ \
-	-pedantic \
 	-Wall \
-	-Wextra \
 	-Wshadow \
 	-Wrestrict \
 	-Wformat=2 \
 	-Wwrite-strings \
-	-Wcast-qual \
 	-Wpointer-arith \
 	-Wstrict-prototypes \
 	-Wmissing-prototypes \
 	-Wmissing-declarations \
 	-Wno-unused-parameter \
+	-Wno-unused-function \
+	-Wno-unused-variable \
+	-Wno-unused-but-set-variable \
+	-Wno-pointer-sign \
+	-Wno-shadow \
 ])
 
 # Windows specific compiler options.