build: detect system libpam in prepare-local-deps

prepare-local-deps.sh only stages PAM headers and expects final linking
against the host libpam.  Some distributions/containers do not provide the
unversioned /usr/lib/libpam.so development symlink, but do provide the runtime
soname such as /lib/x86_64-linux-gnu/libpam.so.0.

Detect the host libpam path when printing the CMake hints so the generated
command can be used directly in those environments.

prepare-local-deps.sh

@@ -24,6 +24,18 @@ find_one() {
     printf '%s\n' "$1"
 }
 
+find_system_lib() {
+    soname=$1
+    for dir in /lib /usr/lib /lib/* /usr/lib/* /usr/local/lib; do
+        if [ -e "$dir/$soname" ]; then
+            printf '%s\n' "$dir/$soname"
+            return 0
+        fi
+    done
+    command -v ldconfig >/dev/null 2>&1 || return 1
+    ldconfig -p 2>/dev/null | awk -v n="$soname" '$1 == n { print $NF; exit }'
+}
+
 require_root() {
     if [ ! -f .gitmodules ] || [ ! -f CMakeLists.txt ]; then
         echo "error: run this script from the mars-nwe root" >&2
@@ -110,6 +122,14 @@ extract_headers() {
 }
 
 print_cmake_hints() {
+    pam_lib=$(find_system_lib libpam.so 2>/dev/null || true)
+    if [ -z "$pam_lib" ]; then
+        pam_lib=$(find_system_lib libpam.so.0 2>/dev/null || true)
+    fi
+    if [ -z "$pam_lib" ]; then
+        pam_lib=/usr/lib/libpam.so
+    fi
+
     cat <<HINTS
 
 Prepared local dependency prefix: $PREFIX
@@ -121,9 +141,9 @@ Use the local GDBM build and extracted PAM/ncurses headers like this:
     -DGDBM_INCLUDE_DIR=$PWD/$PREFIX/include \\
     -DGDBM_LIBRARY=$PWD/$PREFIX/lib/libgdbm.so \\
     -DPAM_INCLUDE_DIR=$PWD/$PREFIX/include \\
-    -DPAM_LIBRARY=/usr/lib/libpam.so
+    -DPAM_LIBRARY=$pam_lib
 
-Adjust PAM_LIBRARY to the real system libpam path on the build host.  PAM and
+PAM_LIBRARY above was auto-detected for this host.  PAM and
 ncurses are not vendored libraries here: only their headers are staged for
 compile checks, and final linking is against the system libraries.
 HINTS