libid3tag: add pkg-config file

Required for meson to find this library in a sane way.

- In OpenWrt 19.07, there were recently updated mpd to its the latest
version available (current 0.21.22) with transition to meson.
Because of that mpd fails to build due to:

Run-time dependency id3tag found: NO (tried pkgconfig)

This commit fixes it

Signed-off-by: Andre Heider <a.heider@gmail.com>
(cherry picked from commit 1b37a1d2a8)
Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>
[added commit message for OpenWrt 19.07]

libs/libid3tag/Makefile

@@ -42,6 +42,11 @@ TARGET_CFLAGS += $(FPIC)
 
 CONFIGURE_VARS += CC="$(TARGET_CC) $(TARGET_CFLAGS)"
 
+define Build/Prepare
+	$(call Build/Prepare/Default)
+	$(SED) 's,@VERSION@,$(PKG_VERSION),g' $(PKG_BUILD_DIR)/id3tag.pc
+endef
+
 define Build/Configure
 	$(call Build/Configure/Default, \
 		--enable-shared \
@@ -56,6 +61,8 @@ define Build/InstallDev
 	$(CP) $(PKG_INSTALL_DIR)/usr/include/id3tag.h $(1)/usr/include/
 	$(INSTALL_DIR) $(1)/usr/lib
 	$(CP) $(PKG_INSTALL_DIR)/usr/lib/libid3tag.so* $(1)/usr/lib/
+	$(INSTALL_DIR) $(1)/usr/lib/pkgconfig/
+	$(CP) $(PKG_BUILD_DIR)/id3tag.pc $(1)/usr/lib/pkgconfig/
 endef
 
 define Package/libid3tag/install

libs/libid3tag/src/id3tag.pc

@@ -0,0 +1,11 @@
+prefix=/usr
+exec_prefix=/usr
+libdir=${exec_prefix}/lib
+includedir=${prefix}/include
+
+Name: id3tag
+Description: ID3 tag reading library
+Requires:
+Version: @VERSION@
+Libs: -L${libdir} -lid3tag -lz
+Cflags: -I${includedir}