Make the --define-prefix feature available on all platforms

Allowing pkg-config to override the prefix variable in .pc files is a
useful feature for making packages relocatable. There's nothing Windows
specific about it.

Freedesktop #63602 (https://bugs.freedesktop.org/show_bug.cgi?id=63602)

main.c

@@ -461,7 +461,6 @@ static const GOptionEntry options_table[] = {
   { "print-requires-private", 0, G_OPTION_FLAG_NO_ARG, G_OPTION_ARG_CALLBACK,
     &output_opt_cb, "print which packages the package requires for static "
     "linking", NULL },
-#ifdef G_OS_WIN32
   { "define-prefix", 0, 0, G_OPTION_ARG_NONE, &define_prefix,
     "try to override the value of prefix for each .pc file found with a "
     "guesstimated value based on the location of the .pc file", NULL },
@@ -472,6 +471,7 @@ static const GOptionEntry options_table[] = {
   { "prefix-variable", 0, 0, G_OPTION_ARG_STRING, &prefix_variable,
     "set the name of the variable that pkg-config automatically sets",
     "PREFIX" },
+#ifdef G_OS_WIN32
   { "msvc-syntax", 0, 0, G_OPTION_ARG_NONE, &msvc_syntax,
     "output -l and -L flags for the Microsoft compiler (cl)", NULL },
 #endif

parse.c

@@ -34,9 +34,10 @@
 #endif
 #include <sys/types.h>
 
-#ifdef G_OS_WIN32
 gboolean define_prefix = ENABLE_DEFINE_PREFIX;
 char *prefix_variable = "prefix";
+
+#ifdef G_OS_WIN32
 gboolean msvc_syntax = FALSE;
 #endif
 
@@ -943,7 +944,6 @@ parse_line (Package *pkg, const char *untrimmed, const char *path,
       if (pkg->vars == NULL)
         pkg->vars = g_hash_table_new (g_str_hash, g_str_equal);
 
-#ifdef G_OS_WIN32
       if (define_prefix && strcmp (tag, prefix_variable) == 0)
 	{
 	  /* This is the prefix variable. Try to guesstimate a value for it
@@ -1006,7 +1006,6 @@ parse_line (Package *pkg, const char *untrimmed, const char *path,
 				 p + strlen (pkg->orig_prefix), NULL);
 	  g_free (oldstr);
 	}
-#endif
 
       if (g_hash_table_lookup (pkg->vars, tag))
         {

pkg-config.1

@@ -211,20 +211,37 @@ for the linker, and should be placed on the cl command line after a
 .I "--define-prefix"
 .TQ
 .I "--dont-define-prefix"
-These options are available only on Windows. They control whether
+These options control whether
 .I pkg-config
-overrides the value of the variable "prefix" in each .pc file. With
-\-\-define-prefix,
+overrides the value of the variable
+.I prefix
+in each .pc file. With \-\-define-prefix,
 .I pkg-config
 uses the installed location of the .pc file to determine the
 prefix. \-\-dont-define-prefix prevents this behavior. The default is
 usually \-\-define-prefix.
+
+When this feature is enabled and a .pc file is found in a directory named
+.IR pkgconfig ,
+the prefix for that package is assumed to be the grandparent of the
+directory where the file was found, and the
+.I prefix
+variable is overridden for that file accordingly.
+
+If the value of a variable in a .pc file begins with the original,
+non-overridden, value of the
+.I prefix
+variable, then the overridden value of
+.I prefix
+is used instead. This allows the feature to work even when the variables
+have been expanded in the .pc file.
 .TP
 .I "--prefix-variable=PREFIX"
-This option is available only on Windows. It sets the name of the
-variable that
+Set the name of the variable that
 .I pkg-config
-overrides instead of "prefix" when using the \-\-define-prefix feature.
+overrides instead of
+.I prefix
+when using the \-\-define-prefix feature.
 .TP
 .I "--static"
 Output libraries suitable for static linking.  That means including
@@ -317,23 +334,6 @@ and
 .IR share\epkgconfig .
 This can be augmented or replaced using the standard environment
 variables described above.
-
-If a .pc file is found in a directory named
-.IR pkgconfig ,
-the prefix for that package is assumed to be the grandparent of the
-directory where the file was found, and the
-.I prefix
-variable is overridden for that file accordingly.
-
-If the value of a variable in a .pc file begins with the original,
-non-overridden, value of the
-.I prefix
-variable, then the overridden value of
-.I prefix
-is used instead. This allows the feature to work even when the variables
-have been expanded in the .pc file.
-
-See the \-\-dont-define-prefix option for disabling this feature.
 .\"
 .SH AUTOCONF MACROS
 .TP

pkg.h

@@ -85,9 +85,7 @@ struct _Package
   int libs_num; /* Number of times the "Libs" header has been seen */
   int libs_private_num;  /* Number of times the "Libs.private" header has been seen */
   gboolean in_requires_chain; /* package is in current Requires chain */
-#ifdef G_OS_WIN32
   char *orig_prefix; /* original prefix value before redefinition */
-#endif
 };
 
 Package *get_package               (const char *name);
@@ -136,13 +134,13 @@ extern char *pcsysrootdir;
  */
 extern char *pkg_config_pc_path;
 
-#ifdef G_OS_WIN32
 /* If TRUE, define "prefix" in .pc files at runtime. */
 extern gboolean define_prefix;
 
 /* The name of the variable that acts as prefix, unless it is "prefix" */
 extern char *prefix_variable;
 
+#ifdef G_OS_WIN32
 /* If TRUE, output flags in MSVC syntax. */
 extern gboolean msvc_syntax;
 #endif