Don't override empty prefix setting

If the original prefix setting is empty, skip prepending the redefined
prefix to other variables. This works the same as if the pc file doesn't
have a prefix variable at all.

https://bugs.freedesktop.org/show_bug.cgi?id=97453-empty

check/Makefile.am

@@ -99,6 +99,7 @@ EXTRA_DIST = \
 	gtk/xproto.pc \
 	gtk/xrender.pc \
 	tilde.pc \
+	pkgconfig/empty-prefix.pc \
 	pkgconfig/prefixdef.pc \
 	pkgconfig/prefixdef-expanded.pc \
 	pcfiledir.pc \

check/check-relocatable

@@ -44,3 +44,10 @@ for pkg in prefixdef prefixdef-expanded; do
     RESULT="-I/reloc/include -L/reloc/lib -lfoo"
     run_test --dont-define-prefix --cflags --libs $pkg
 done
+
+# Test prefix redefinition for .pc files with an empty prefix. In this
+# case, there should be no prefix adjustment to the other variables. The
+# result should be the same regardless of prefix redefinition.
+RESULT="-I/some/path/include -L/some/path/lib -lfoo"
+run_test --define-prefix --cflags --libs empty-prefix
+run_test --dont-define-prefix --cflags --libs empty-prefix

check/pkgconfig/empty-prefix.pc

@@ -0,0 +1,10 @@
+prefix=
+libdir=/some/path/lib
+includedir=/some/path/include
+
+Name: Empty prefix redefinition test
+Description: Test pkg-config empty prefix redefinition
+Version: 1.0.0
+Requires:
+Libs: -L${libdir} -lfoo
+Cflags: -I${includedir}

parse.c

@@ -1046,6 +1046,7 @@ parse_line (Package *pkg, const char *untrimmed, const char *path,
 	}
       else if (define_prefix &&
 	       pkg->orig_prefix != NULL &&
+	       *(pkg->orig_prefix) != '\0' &&
 	       strncmp (p, pkg->orig_prefix, strlen (pkg->orig_prefix)) == 0 &&
 	       G_IS_DIR_SEPARATOR (p[strlen (pkg->orig_prefix)]))
 	{