Merge Using destdir for .pc install

This merge allows us to use the `DESTDIR` environment variable when installing the pkg config file. All other parts of the install step already handle that variable correctly. Also uses the path of the installed config file to deduce the pkg prefix.

Related PR: #1597

cmake/generate_pkg.cmake.in

@@ -12,13 +12,16 @@ string(REPLACE " " "\ " GINKGO_PKG_INSTALL_PREFIX "${CMAKE_INSTALL_PREFIX}")
 string(REPLACE ";" "\ " GINKGO_INTERFACE_LINK_FLAGS "@GINKGO_INTERFACE_LINK_FLAGS@")
 string(REPLACE ";" "\ " GINKGO_INTERFACE_CXX_FLAGS "@GINKGO_INTERFACE_CXX_FLAGS@")
 
+# @TODO: better to use file(REAL_PATH ...) with cmake 3.19+
+get_filename_component(GINKGO_PKG_INSTALL_PREFIX ${GINKGO_PKG_INSTALL_PREFIX} ABSOLUTE)
+
 set(GINKGO_INSTALL_LIBDIR "@CMAKE_INSTALL_LIBDIR@")
 set(GINKGO_INSTALL_INCLUDEDIR "@CMAKE_INSTALL_INCLUDEDIR@")
 set(GINKGO_PROJECT_NAME "@CMAKE_PROJECT_NAME@")
 set(GINKGO_DESCRIPTION "@Ginkgo_DESCRIPTION@")
 set(GINKGO_VERSION "@Ginkgo_VERSION@")
 
-set(GINKGO_INSTALL_PKGCONFIG_DIR "${GINKGO_PKG_INSTALL_PREFIX}/${GINKGO_INSTALL_LIBDIR}/pkgconfig")
+set(GINKGO_INSTALL_PKGCONFIG_DIR "$ENV{DESTDIR}${GINKGO_PKG_INSTALL_PREFIX}/${GINKGO_INSTALL_LIBDIR}/pkgconfig")
 
 set(GINKGO_SOURCE_DIR "@Ginkgo_SOURCE_DIR@")
 set(GINKGO_BINARY_DIR "@Ginkgo_BINARY_DIR@")

cmake/ginkgo.pc.in

@@ -1,4 +1,4 @@
-prefix=@GINKGO_PKG_INSTALL_PREFIX@
+prefix=${pcfiledir}/../..
 libdir=${prefix}/@GINKGO_INSTALL_LIBDIR@
 includedir=${prefix}/@GINKGO_INSTALL_INCLUDEDIR@