Fix debug postfix in pkgconfig file

In case of USE_POSTFIX, the POSTFIX is dependent
on the CMAKE_BUILD_TYPE.
Use the correct POSTFIX also in the generated pugixml.pc file.

This results in the following contents of pugixml.pc:
- Release:
  Libs: -L${libdir} -lpugixml
- RelWithDebInfo
  Libs: -L${libdir} -lpugixml_r
- MinSizeRel:
  Libs: -L${libdir} -lpugixml_m
- Debug:
  Libs: -L${libdir} -lpugixml_d

CMakeLists.txt

@@ -142,6 +142,16 @@ write_basic_package_version_file(
   "${PROJECT_BINARY_DIR}/pugixml-config-version.cmake"
   COMPATIBILITY SameMajorVersion)
 
+if (USE_POSTFIX)
+  if(CMAKE_BUILD_TYPE MATCHES RelWithDebInfo)
+    set(LIB_POSTFIX ${CMAKE_RELWITHDEBINFO_POSTFIX})
+  elseif(CMAKE_BUILD_TYPE MATCHES MinSizeRel)
+    set(LIB_POSTFIX ${CMAKE_MINSIZEREL_POSTFIX})
+  elseif(CMAKE_BUILD_TYPE MATCHES Debug)
+    set(LIB_POSTFIX ${CMAKE_DEBUG_POSTFIX})
+  endif()
+endif()
+
 configure_file(scripts/pugixml.pc.in pugixml.pc @ONLY)
 
 if (NOT DEFINED PUGIXML_RUNTIME_COMPONENT)

scripts/pugixml.pc.in

@@ -8,4 +8,4 @@ Description: Light-weight, simple and fast XML parser for C++ with XPath support
 URL: https://pugixml.org/
 Version: @pugixml_VERSION@
 Cflags: -I${includedir}
-Libs: -L${libdir} -lpugixml
+Libs: -L${libdir} -lpugixml@LIB_POSTFIX@