Improve handling of gflags library

Set minimun gflags version to 2.1.2

Replaced google:: by gflags:: namespace

cmake/Modules/FindGFLAGS.cmake

@@ -115,10 +115,7 @@ else()
     set(GFlags_FOUND false)
 endif()
 
-include(FindPackageHandleStandardArgs)
-find_package_handle_standard_args(GFLAGS DEFAULT_MSG GFlags_LIBS GFlags_INCLUDE_DIRS)
-
-if(GFLAGS_FOUND)
+if(GFlags_LIBS AND GFlags_INCLUDE_DIRS)
     if(NOT PACKAGE_VERSION)
         set(PACKAGE_VERSION "")
     endif()
@@ -129,6 +126,11 @@ if(GFLAGS_FOUND)
     if(EXISTS ${GFlags_LIBS_DIR}/cmake/gflags/gflags-config-version.cmake)
         include(${GFlags_LIBS_DIR}/cmake/gflags/gflags-config-version.cmake)
     endif()
+    if(NOT PACKAGE_VERSION)
+        if(EXISTS ${GFlags_INCLUDE_DIRS}/google/gflags.h)
+            set(PACKAGE_VERSION "2.0")
+        endif()
+    endif()
     if(PACKAGE_VERSION)
         set(GFLAGS_VERSION ${PACKAGE_VERSION})
         set_package_properties(GFLAGS PROPERTIES
@@ -150,6 +152,17 @@ set_package_properties(GFLAGS PROPERTIES
     URL "https://github.com/gflags/gflags"
 )
 
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(GFLAGS DEFAULT_MSG GFlags_LIBS GFlags_INCLUDE_DIRS)
+
+if(GFLAGS_VERSION)
+    if(${GFLAGS_VERSION} VERSION_LESS "${GNSSSDR_GFLAGS_MIN_VERSION}")
+        set(GFLAGS_FOUND FALSE)
+        unset(GFlags_LIBS CACHE)
+        unset(GFlags_INCLUDE_DIRS CACHE)
+    endif()
+endif()
+
 if(GFLAGS_FOUND AND NOT TARGET Gflags::gflags)
     add_library(Gflags::gflags SHARED IMPORTED)
     set_target_properties(Gflags::gflags PROPERTIES