Correctly set symbol file header when using gcc on Windows.

Change-Id: I624962cfa990faf37c3ca4b62a65f6b0f6f91ad6

build/cmake/generate_exports.cmake

diff --git a/build/cmake/generate_exports.cmake b/build/cmake/generate_exports.cmake
index 8ab4a7a..964804b 100644
--- a/build/cmake/generate_exports.cmake
+++ b/build/cmake/generate_exports.cmake
@@ -33,7 +33,7 @@
 
 if("${AOM_TARGET_SYSTEM}" STREQUAL "Darwin")
   file(REMOVE "${aom_sym_file}")
-elseif("${AOM_TARGET_SYSTEM}" MATCHES "Windows\|MSYS")
+elseif("${AOM_TARGET_SYSTEM}" MATCHES "Windows\|MSYS" AND AOM_MSVC)
   file(WRITE "${aom_sym_file}" "LIBRARY aom\n" "EXPORTS\n")
 else()
   file(WRITE "${aom_sym_file}" "{ global:\n")