Use CPP, CC and flags in dep check scripts

Allow build systems without standard cc to successfully run the
dependency checking helper scripts from the Makefile.
This supports custom compilers specified by the common CC environment
variable, preprocessors given as CPP and additional preprocessor flags
from CPPFLAGS.
Additional flags from CFLAGS and LDFLAGS are considered for compiling/linking.
Overall, this facilitates cross-compilation and similar setups.

Signed-off-by: Marcel Bargull <marcel.bargull@udo.edu>

hack/btrfs_installed_tag.sh

@@ -1,5 +1,5 @@
 #!/usr/bin/env bash
-cc -E - > /dev/null 2> /dev/null << EOF
+${CPP:-${CC:-cc} -E} ${CPPFLAGS} - > /dev/null 2> /dev/null << EOF
 #include <btrfs/ioctl.h>
 EOF
 if test $? -ne 0 ; then

hack/btrfs_tag.sh

@@ -1,5 +1,5 @@
 #!/usr/bin/env bash
-cc -E - > /dev/null 2> /dev/null << EOF
+${CPP:-${CC:-cc} -E} ${CPPFLAGS} - > /dev/null 2> /dev/null << EOF
 #include <btrfs/version.h>
 EOF
 if test $? -ne 0 ; then

hack/libdm_tag.sh

@@ -2,7 +2,7 @@
 tmpdir="$PWD/tmp.$RANDOM"
 mkdir -p "$tmpdir"
 trap 'rm -fr "$tmpdir"' EXIT
-cc -o "$tmpdir"/libdm_tag -ldevmapper -x c - > /dev/null 2> /dev/null << EOF
+${CC:-cc} ${CFLAGS} ${CPPFLAGS} ${LDFLAGS} -o "$tmpdir"/libdm_tag -x c - -ldevmapper > /dev/null 2> /dev/null << EOF
 #include <libdevmapper.h>
 int main() {
 	struct dm_task *task;

hack/systemd_tag.sh

@@ -1,5 +1,5 @@
 #!/usr/bin/env bash
-cc -E - > /dev/null 2> /dev/null << EOF
+${CPP:-${CC:-cc} -E} ${CPPFLAGS} - > /dev/null 2> /dev/null << EOF
 #include <systemd/sd-daemon.h>
 EOF
 if test $? -eq 0 ; then