PARQUET-962: [C++] GTEST_MAIN_STATIC_LIB is not defined in FindGTest.cmake

Author: Deepak Majeti <deepak.majeti@hpe.com>

Closes #306 from majetideepak/PARQUET-962 and squashes the following commits:

07926af [Deepak Majeti] mark as advanced
74a62b9 [Deepak Majeti] define GTEST_MAIN_STATIC_LIB in FindGTest.cmake
diff --git a/cmake_modules/FindGTest.cmake b/cmake_modules/FindGTest.cmake
index 3c5d2b6..c6a4238 100644
--- a/cmake_modules/FindGTest.cmake
+++ b/cmake_modules/FindGTest.cmake
@@ -28,6 +28,7 @@
 #  GTEST_INCLUDE_DIR, directory containing headers
 #  GTEST_LIBS, directory containing gtest libraries
 #  GTEST_STATIC_LIB, path to libgtest.a
+#  GTEST_MAIN_STATIC_LIB, path to libgtest_main.a
 #  GTEST_SHARED_LIB, path to libgtest's shared library
 #  GTEST_FOUND, whether gtest has been found
 
@@ -43,7 +44,7 @@
     find_path( GTEST_INCLUDE_DIR NAMES gtest/gtest.h
         PATHS ${_gtest_roots} NO_DEFAULT_PATH
         PATH_SUFFIXES "include" )
-    find_library( GTEST_LIBRARIES NAMES gtest
+    find_library( GTEST_LIBRARIES NAMES gtest gtest_main
         PATHS ${_gtest_roots} NO_DEFAULT_PATH
         PATH_SUFFIXES "lib" )
 else ()
@@ -57,6 +58,7 @@
   get_filename_component( GTEST_LIBS ${GTEST_LIBRARIES} PATH )
   set(GTEST_LIB_NAME libgtest)
   set(GTEST_STATIC_LIB ${GTEST_LIBS}/${GTEST_LIB_NAME}.a)
+  set(GTEST_MAIN_STATIC_LIB ${GTEST_LIBS}/${GTEST_LIB_NAME}_main.a)
   set(GTEST_SHARED_LIB ${GTEST_LIBS}/${GTEST_LIB_NAME}${CMAKE_SHARED_LIBRARY_SUFFIX})
 else ()
   set(GTEST_FOUND FALSE)
@@ -87,5 +89,6 @@
   GTEST_LIBS
   GTEST_LIBRARIES
   GTEST_STATIC_LIB
+  GTEST_MAIN_STATIC_LIB
   GTEST_SHARED_LIB
 )