Do not add -lm to Libs.private if WIN32 or APPLE

In Windows and Apple operating systems, the functions declared in
<math.h> are defined in the main standard C library rather than a
separate libm. Do not add -lm to the Libs.private field in aom.pc if
WIN32 or APPLE is true.

This condition matches the condition under which we link the aom and
aom_av1_rc libraries with -lm in CMakeLists.txt.

Bug: 356153293
Change-Id: Ic42c84b0583aafa4491e393d691939c293415986
(cherry picked from commit 74c99dfe6d63535b9a033322c85581391eba913c)

build/cmake/pkg_config.cmake

diff --git a/build/cmake/pkg_config.cmake b/build/cmake/pkg_config.cmake
index c4f9480..a229c00 100644
--- a/build/cmake/pkg_config.cmake
+++ b/build/cmake/pkg_config.cmake
@@ -60,10 +60,11 @@
 endif()
 file(APPEND "${pkgconfig_file}" "\nConflicts:\n")
 file(APPEND "${pkgconfig_file}" "Libs: -L\${libdir} -l${pkg_name}\n")
-if(CONFIG_MULTITHREAD AND CMAKE_THREAD_LIBS_INIT)
-  file(APPEND "${pkgconfig_file}"
-       "Libs.private: -lm ${CMAKE_THREAD_LIBS_INIT}\n")
-else()
-  file(APPEND "${pkgconfig_file}" "Libs.private: -lm\n")
+file(APPEND "${pkgconfig_file}" "Libs.private:")
+if(NOT WIN32 AND NOT APPLE)
+  file(APPEND "${pkgconfig_file}" " -lm")
 endif()
-file(APPEND "${pkgconfig_file}" "Cflags: -I\${includedir}\n")
+if(CONFIG_MULTITHREAD AND CMAKE_THREAD_LIBS_INIT)
+  file(APPEND "${pkgconfig_file}" " ${CMAKE_THREAD_LIBS_INIT}")
+endif()
+file(APPEND "${pkgconfig_file}" "\nCflags: -I\${includedir}\n")