configure: List the env vars needed if pkg-config doesn't work

Part of 31922.

configure.ac

@@ -30,11 +30,11 @@ AC_CANONICAL_HOST
 
 PKG_PROG_PKG_CONFIG
 if test "x$PKG_CONFIG" = "x" ; then
-    pkg_config_user_action="install pkg-config, and check the PKG_CONFIG_PATH environment variable."
+    pkg_config_user_action="install pkg-config, and check the PKG_CONFIG_PATH environment variable"
     AC_MSG_NOTICE([Some libraries need pkg-config, including systemd, nss, lzma, zstd, and custom mallocs.])
-    AC_MSG_NOTICE([To use those libraries, $pkg_config_user_action])
+    AC_MSG_NOTICE([To use those libraries, $pkg_config_user_action.])
 else
-    pkg_config_user_action="check the PKG_CONFIG_PATH environment variable."
+    pkg_config_user_action="check the PKG_CONFIG_PATH environment variable"
 fi
 
 AC_ARG_ENABLE(openbsd-malloc,
@@ -173,7 +173,7 @@ AC_SUBST(TOR_SYSTEMD_CFLAGS)
 AC_SUBST(TOR_SYSTEMD_LIBS)
 
 if test "x$enable_systemd" = "xyes" -a "x$have_systemd" != "xyes" ; then
-    AC_MSG_ERROR([Explicitly requested systemd support, but systemd not found, $pkg_config_user_action])
+    AC_MSG_ERROR([Explicitly requested systemd support, but systemd not found, $pkg_config_user_action, or set SYSTEMD_CFLAGS and SYSTEMD_LIBS.])
 fi
 
 case "$host" in
@@ -877,7 +877,7 @@ if test "x$enable_nss" = "xyes"; then
   PKG_CHECK_MODULES(NSS,
      [nss],
      [have_nss=yes],
-     [have_nss=no; AC_MSG_ERROR([You asked for NSS but I can't find it, $pkg_config_user_action])])
+     [have_nss=no; AC_MSG_ERROR([You asked for NSS but I can't find it, $pkg_config_user_action, or set NSS_CFLAGS and NSS_LIBS.])])
   AC_SUBST(NSS_CFLAGS)
   AC_SUBST(NSS_LIBS)
 fi
@@ -1083,7 +1083,7 @@ else
                       have_lzma=no)
 
     if test "x$have_lzma" = "xno" ; then
-        AC_MSG_WARN([Unable to find liblzma, $pkg_config_user_action])
+        AC_MSG_WARN([Unable to find liblzma, $pkg_config_user_action, or set LZMA_CFLAGS and LZMA_LIBS.])
     fi
 fi
 
@@ -1115,7 +1115,7 @@ else
                       have_zstd=no)
 
     if test "x$have_zstd" = "xno" ; then
-        AC_MSG_WARN([Unable to find libzstd, $pkg_config_user_action])
+        AC_MSG_WARN([Unable to find libzstd, $pkg_config_user_action, or set ZSTD_CFLAGS and ZSTD_LIBS.])
     fi
 fi
 
@@ -1922,7 +1922,7 @@ AS_CASE([$malloc],
 			have_tcmalloc=no)
 
       if test "x$have_tcmalloc" = "xno" ; then
-          AC_MSG_ERROR([Unable to find tcmalloc requested by --with-malloc, $pkg_config_user_action])
+          AC_MSG_ERROR([Unable to find tcmalloc requested by --with-malloc, $pkg_config_user_action, or set TCMALLOC_CFLAGS and TCMALLOC_LIBS.])
       fi
 
       CFLAGS="$CFLAGS $TCMALLOC_CFLAGS"
@@ -1936,7 +1936,7 @@ AS_CASE([$malloc],
 			have_jemalloc=no)
 
       if test "x$have_tcmalloc" = "xno" ; then
-          AC_MSG_ERROR([Unable to find jemalloc requested by --with-malloc, $pkg_config_user_action])
+          AC_MSG_ERROR([Unable to find jemalloc requested by --with-malloc, $pkg_config_user_action, or set JEMALLOC_CFLAGS and JEMALLOC_LIBS.])
       fi
 
       CFLAGS="$CFLAGS $JEMALLOC_CFLAGS"