fix generated libmsh3.pc file (#225)

* libmsh3.pc.in: fix include path in `Cflags:`

* libmsh3.pc.in: set the actual `prefix:` value

* libmsh3.pc.in: do not substitute macros (with empty values)

lib/CMakeLists.txt

@@ -29,7 +29,8 @@ install(FILES ../msh3.h DESTINATION include)
 configure_file(msh3-config.cmake.in ${CMAKE_BINARY_DIR}/msh3-config.cmake)
 install(FILES ${CMAKE_BINARY_DIR}/msh3-config.cmake DESTINATION share/msh3)
 install(EXPORT msh3 DESTINATION share/msh3)
-configure_file(libmsh3.pc.in ${CMAKE_BINARY_DIR}/libmsh3.pc)
+set(prefix ${CMAKE_INSTALL_PREFIX})
+configure_file(libmsh3.pc.in ${CMAKE_BINARY_DIR}/libmsh3.pc @ONLY)
 if(NOT WIN32)
     install(FILES ${CMAKE_BINARY_DIR}/libmsh3.pc DESTINATION lib/pkgconfig)
 endif()

lib/libmsh3.pc.in

@@ -1,7 +1,7 @@
 # Copyright (c) Microsoft Corporation.
 # Licensed under the MIT License.
 
-prefix=/usr/local
+prefix=@prefix@
 exec_prefix=${prefix}
 libdir=${exec_prefix}/lib
 includedir=${exec_prefix}/include
@@ -10,4 +10,4 @@ Name: libmsh3
 Description: Minimal HTTP/3 client on top of MsQuic
 Version: 0.1.0
 Libs: -L${libdir} -lmsh3
-Cflags: -I${includedir}/libmsh3
+Cflags: -I${includedir}