configure.sh: allow passing pkg-config to use via HOST_PKG_CONFIG

In nixpkgs we sometimes prefix pkg-config (e. g. when cross compiling),
so different versions, returning library information for different
targets can be disambiguated. For this to work properly with
configure.sh we need some way to pass in the pkg-config binary to
use. Naming follows the target logic: HOST_PKG_CONFIG returns libraries
for whatever target HOST is, that is libraries HOST_CC/LD will link
executables against.

configure.sh

@@ -60,6 +60,10 @@ Environment variables affecting the build system configuration:
     HOST_AR
         C compiler and ar tool used for host tools and tenders.
 
+    HOST_PKG_CONFIG
+        pkg-config executable to query for information on libraries needed to
+        build host tools and tenders.
+
     TARGET_CC
     TARGET_LD
     TARGET_OBJCOPY
@@ -284,6 +288,8 @@ case ${HOST_CC_MACHINE} in
         ;;
 esac
 
+HOST_PKG_CONFIG=${HOST_PKG_CONFIG:-pkg-config}
+
 CONFIG_SPT_TENDER_NO_PIE=
 CONFIG_SPT_TENDER_LIBSECCOMP_CFLAGS=
 CONFIG_SPT_TENDER_LIBSECCOMP_LDFLAGS=
@@ -298,24 +304,24 @@ if [ -n "${CONFIG_SPT_TENDER}" ]; then
         CONFIG_SPT_TENDER_NO_PIE=1
     fi
 
-    if ! command -v pkg-config >/dev/null; then
-        die "pkg-config is required"
+    if ! command -v $HOST_PKG_CONFIG >/dev/null; then
+        die "pkg-config is required: $HOST_PKG_CONFIG not in PATH"
     fi
-    if ! pkg-config libseccomp; then
+    if ! $HOST_PKG_CONFIG libseccomp; then
         die "libseccomp development headers are required"
     else
-        if ! pkg-config --atleast-version=2.3.3 libseccomp; then
+        if ! $HOST_PKG_CONFIG --atleast-version=2.3.3 libseccomp; then
             # TODO Make this a hard error once there are no distros with
             # libseccomp < 2.3.3 in the various CIs.
             warn "libseccomp >= 2.3.3 is required" \
                 "for correct spt tender operation"
             warn "Proceeding anyway, expect tests to fail"
-        elif ! pkg-config --atleast-version=2.4.1 libseccomp; then
+        elif ! $HOST_PKG_CONFIG --atleast-version=2.4.1 libseccomp; then
             warn "libseccomp < 2.4.1 has known vulnerabilities"
             warn "Proceeding anyway, but consider upgrading"
         fi
-        CONFIG_SPT_TENDER_LIBSECCOMP_CFLAGS="$(pkg-config --cflags libseccomp)"
-        CONFIG_SPT_TENDER_LIBSECCOMP_LDLIBS="$(pkg-config --libs libseccomp)"
+        CONFIG_SPT_TENDER_LIBSECCOMP_CFLAGS="$($HOST_PKG_CONFIG --cflags libseccomp)"
+        CONFIG_SPT_TENDER_LIBSECCOMP_LDLIBS="$($HOST_PKG_CONFIG --libs libseccomp)"
     fi
     if ! CC="${HOST_CC}" PKG_CFLAGS="${CONFIG_SPT_TENDER_LIBSECCOMP_CFLAGS}" \
         cc_check_header seccomp.h; then