Remove C99 requirement from CMake file (#633)

* Remove C99 requirement from CMake file

The kernel source is C89 compliant and does not need C99.

Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>

* Explicitly set C89 requirement for kernel

Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>

---------

Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index aead9a0..b51b7aa 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -230,8 +230,7 @@
 
 ########################################################################
 # Requirements
-set(CMAKE_C_STANDARD 99)
-set(CMAKE_C_STANDARD_REQUIRED ON)
+set_property(TARGET freertos_kernel PROPERTY C_STANDARD 90)
 
 ########################################################################
 # Overall Compile Options