Changes

Revert `configure: regular_C(PP)FLAGS -> C(PP)FLAGS`

This commit reverts dd335ef8fc41567e324a3d26f1ec30d04c69a0bb.

Observed:

```
$ configure && make CPPFLAGS=-DNDEBUG
gcc -DHAVE_CONFIG_H -I. -I..  -DNDEBUG -I../include -I/usr/include/libmnl -DNDEBUG -Wall -Waggregate-return -Wmissing-declarations     -Wmissing-prototypes -Wshadow -Wstrict-prototypes       -Wformat=2 -pipe -std=gnu11 -fvisibility=hidden -Wall -Waggregate-return -Wmissing-declarations        -Wmissing-prototypes -Wshadow -Wstrict-prototypes       -Wformat=2 -pipe -std=gnu11 -MT gtp-tunnel.o -MD -MP -MF $depbase.Tpo -c -o gtp-tunnel.o gtp-tunnel.c &&\
```

Expected:

```
gcc -DHAVE_CONFIG_H -I. -I..  -D_FILE_OFFSET_BITS=64 -D_REENTRANT -I../include -I/usr/include/libmnl -D_FILE_OFFSET_BITS=64 -D_REENTRANT -Wall -Waggregate-return -Wmissing-declarations       -Wmissing-prototypes -Wshadow -Wstrict-prototypes      -Wformat=2 -pipe -std=gnu11 -fvisibility=hidden -Wall -Waggregate-return -Wmissing-declarations         -Wmissing-prototypes -Wshadow -Wstrict-prototypes      -Wformat=2 -pipe -std=gnu11 -MT gtp-tunnel.o -MD -MP -MF $depbase.Tpo -c -o gtp-tunnel.o gtp-tunnel.c &&\
```

Including ${CFLAGS} in AM_CFLAGS (which is totally wrong)
is the cause for the double apperance of -DNDEBUG.

_FILE_OFFSET_BITS is lost because dd335 violated one core principle
of autoconf/automake: configure MUST run idempotent and not touch
CPPFLAGS and CFLAGS, pursuant to automake.info §3.6 "User variables".

Change-Id: I5cc30885cec564346997269b51decbbe49548ddf

build: fix bashims in configure.ac

$ foo=bar
$ foo+=zzz
dash: 2: foo+=zzz: not found

Fixes: 1.2.5-6-g0bd1e0d
Change-Id: I2c0a62ab80f480502dd2a1bcf4aef71e70152ea6