build: stop overriding CFLAGS

CFLAGS is reserved for the user. configure must finish in an
idempotent state and not touch it, pursuant to automake.info §3.6
"Variables reserved for the user".

Observed:

```
$ ./configure && make CFLAGS=-O1
…
libtool: compile:  gcc -DHAVE_CONFIG_H -I. -Wall -I..
-I./src/sss_client -I./src -I. -I/usr/include/samba-4.0
-I/usr/include/dbus-1.0 -I/usr/lib64/dbus-1.0/include
-I/usr/include/libnl3 -DLIBDIR=\"/usr/local/lib\"
-DVARDIR=\"/usr/local/var\" -DRUNDIR=\"/usr/local/var/run\"
-DSSS_STATEDIR=\"/usr/local/var/lib/sss\"
-DSYSCONFDIR=\"/usr/local/etc\" -DSHLIBEXT=\"\"
-DSSSDDATADIR=\"/usr/local/share/sssd\"
-DSSSD_LIBEXEC_PATH=\"/usr/local/libexec/sssd\"
-DSSSD_CONF_DIR=\"/usr/local/etc/sssd\"
-DSSS_NSS_MCACHE_DIR=\"/usr/local/var/lib/sss/mc\"
-DSSS_NSS_SOCKET_NAME=\"/usr/local/var/lib/sss/pipes/nss\"
-DSSS_PAM_SOCKET_NAME=\"/usr/local/var/lib/sss/pipes/pam\"
-DSSS_PAC_SOCKET_NAME=\"/usr/local/var/lib/sss/pipes/pac\"
-DSSS_SUDO_SOCKET_NAME=\"/usr/local/var/lib/sss/pipes/sudo\"
-DSSS_AUTOFS_SOCKET_NAME=\"/usr/local/var/lib/sss/pipes/autofs\"
-DSSS_SSH_SOCKET_NAME=\"/usr/local/var/lib/sss/pipes/ssh\"
-DLOCALEDIR=\"/usr/local/share/locale\"
-DBASE_FILE_STEM=\"libsss_util_la-sysdb_ops\" -Wall -Wshadow
-Wstrict-prototypes -Wpointer-arith -Wcast-qual -Wcast-align
-Wwrite-strings -Wundef -Werror-implicit-function-declaration
-Winit-self -Wmissing-include-dirs -fno-strict-aliasing -std=gnu99
-O1 -MT src/db/libsss_util_la-sysdb_ops.lo -MD -MP -MF
src/db/.deps/libsss_util_la-sysdb_ops.Tpo -c src/db/sysdb_ops.c -fPIC
-DPIC -o src/db/.libs/libsss_util_la-sysdb_ops.o
```

Expected:

```
libtool: compile:  gcc -DHAVE_CONFIG_H -I. -Wall -I..
-I./src/sss_client -I./src -I. -I/usr/include/samba-4.0
-I/usr/include/dbus-1.0 -I/usr/lib64/dbus-1.0/include
-I/usr/include/libnl3 -DLIBDIR=\"/usr/local/lib\"
-DVARDIR=\"/usr/local/var\" -DRUNDIR=\"/usr/local/var/run\"
-DSSS_STATEDIR=\"/usr/local/var/lib/sss\"
-DSYSCONFDIR=\"/usr/local/etc\" -DSHLIBEXT=\"\"
-DSSSDDATADIR=\"/usr/local/share/sssd\"
-DSSSD_LIBEXEC_PATH=\"/usr/local/libexec/sssd\"
-DSSSD_CONF_DIR=\"/usr/local/etc/sssd\"
-DSSS_NSS_MCACHE_DIR=\"/usr/local/var/lib/sss/mc\"
-DSSS_NSS_SOCKET_NAME=\"/usr/local/var/lib/sss/pipes/nss\"
-DSSS_PAM_SOCKET_NAME=\"/usr/local/var/lib/sss/pipes/pam\"
-DSSS_PAC_SOCKET_NAME=\"/usr/local/var/lib/sss/pipes/pac\"
-DSSS_SUDO_SOCKET_NAME=\"/usr/local/var/lib/sss/pipes/sudo\"
-DSSS_AUTOFS_SOCKET_NAME=\"/usr/local/var/lib/sss/pipes/autofs\"
-DSSS_SSH_SOCKET_NAME=\"/usr/local/var/lib/sss/pipes/ssh\"
-DLOCALEDIR=\"/usr/local/share/locale\"
-DBASE_FILE_STEM=\"libsss_util_la-sysdb_ops\" -Wall -Wshadow
-Wstrict-prototypes -Wpointer-arith -Wcast-qual -Wcast-align
-Wwrite-strings -Wundef -Werror-implicit-function-declaration
-Winit-self -Wmissing-include-dirs -fno-strict-aliasing -std=gnu99
-O1 -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE
-MT src/db/libsss_util_la-sysdb_ops.lo -MD -MP -MF
src/db/.deps/libsss_util_la-sysdb_ops.Tpo -c
```

Fixes: sssd-1_3_0-3-g551aa6c36

Reviewed-by: Iker Pedrosa <[email protected]>
Reviewed-by: Pavel Březina <[email protected]>

Makefile.am

@@ -137,7 +137,7 @@ ifp_non_root_owner_policy =
 endif
 
 
-AM_CFLAGS =
+AM_CFLAGS = $(my_CFLAGS)
 if WANT_AUX_INFO
     AM_CFLAGS += -aux-info $@.X
 endif

configure.ac

@@ -11,7 +11,8 @@ m4_ifdef([AC_USE_SYSTEM_EXTENSIONS],
     [AC_USE_SYSTEM_EXTENSIONS],
     [AC_GNU_SOURCE])
 
-CFLAGS="$CFLAGS -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE"
+my_CFLAGS="-D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE"
+AC_SUBST([my_CFLAGS])
 
 
 AM_INIT_AUTOMAKE([-Wall -Wno-portability foreign subdir-objects tar-pax

src/tests/cwrap/Makefile.am

@@ -22,6 +22,7 @@ AM_CPPFLAGS = \
     $(OPENLDAP_CFLAGS) \
     $(GLIB2_CFLAGS) \
     $(NULL)
+AM_CFLAGS = $(my_CFLAGS)
 
 TESTS_ENVIRONMENT = \
     CWRAP_TEST_SRCDIR=$(abs_srcdir) \

src/tests/intg/Makefile.am

@@ -1,3 +1,4 @@
+AM_CFLAGS = $(my_CFLAGS)
 dist_noinst_DATA = \
     __init__.py \
     config.py.m4 \