cmake: Fix pkgconfig path relocation in mingw

This fixes patch relocation of the output of pkgconfig by adding
missing keywords like prefix, exec_prefix, libdir and includedir.
The pkgconfig output changes are like following:

* Before:
$ pkg-config -libs libssh
-lssh

* After:
$ pkg-config -libs libssh
-LF:/msys64/ucrt64/lib -lssh

See https://people.freedesktop.org/~dbn/pkg-config-guide.html for
further documentation.

Signed-off-by: Biswapriyo Nath <nathbappai@gmail.com>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>
(cherry picked from commit 425353d986)

CMakeLists.txt

@@ -125,7 +125,7 @@ add_subdirectory(src)
 
 # pkg-config file
 if (UNIX OR MINGW)
-configure_file(libssh.pc.cmake ${CMAKE_CURRENT_BINARY_DIR}/libssh.pc)
+configure_file(libssh.pc.cmake ${CMAKE_CURRENT_BINARY_DIR}/libssh.pc @ONLY)
 install(
   FILES
     ${CMAKE_CURRENT_BINARY_DIR}/libssh.pc

libssh.pc.cmake

@@ -1,6 +1,10 @@
-Name: ${PROJECT_NAME}
-Description: The SSH Library
-Version: ${PROJECT_VERSION}
-Libs: -L${CMAKE_INSTALL_FULL_LIBDIR} -lssh
-Cflags: -I${CMAKE_INSTALL_FULL_INCLUDEDIR}
+prefix=@CMAKE_INSTALL_PREFIX@
+exec_prefix=${prefix}
+libdir=@CMAKE_INSTALL_FULL_LIBDIR@
+includedir=@CMAKE_INSTALL_FULL_INCLUDEDIR@
 
+Name: @PROJECT_NAME@
+Description: The SSH Library
+Version: @PROJECT_VERSION@
+Libs: -L${libdir} -lssh
+Cflags: -I${includedir}