fix build when libjpeg is not in its default location (#8172)

coders/Makefile.am

@@ -1025,7 +1025,7 @@ coders_json_la_LIBADD      = $(MAGICKCORE_LIBS)
 
 # JXL coder module
 coders_jxl_la_SOURCES      = coders/jxl.c
-coders_jxl_la_CPPFLAGS     = $(MAGICK_CODER_CPPFLAGS)
+coders_jxl_la_CPPFLAGS     = $(MAGICK_CODER_CPPFLAGS) $(JXL_CFLAGS)
 coders_jxl_la_LDFLAGS      = $(MODULECOMMONFLAGS)
 coders_jxl_la_LIBADD       = $(MAGICKCORE_LIBS) $(JXL_LIBS)
 

configure.ac

@@ -2470,7 +2470,9 @@ if test "$with_jpeg" != 'no'; then
             AC_MSG_RESULT([no -- some components failed test])
             have_jpeg='no (failed tests)'
         else
-            JPEG_LIBS='-ljpeg'
+            if test -z "$JPEG_LIBS"; then
+              JPEG_LIBS='-ljpeg'
+            fi
             LIBS="$JPEG_LIBS $LIBS"
             AC_DEFINE([JPEG_DELEGATE],[1],[Define if you have JPEG library])
             AC_MSG_RESULT([yes])