Modules: Fix typos in name of `CMAKE_COMPILER_IS_GNUCC` variable

It is not called `CMAKE_COMPILER_IS_GNUC` (without last `C`).

Closes: #16297

CompileFlags.cmake

@@ -64,7 +64,7 @@ endif()
 
 # Workaround for short jump tables on PA-RISC
 if(CMAKE_SYSTEM_PROCESSOR MATCHES "^parisc")
-  if(CMAKE_COMPILER_IS_GNUC)
+  if(CMAKE_COMPILER_IS_GNUCC)
     set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -mlong-calls")
   endif()
   if(CMAKE_COMPILER_IS_GNUCXX)

Modules/GenerateExportHeader.cmake

@@ -208,7 +208,7 @@ macro(_test_compiler_hidden_visibility)
 
   if(CMAKE_COMPILER_IS_GNUCXX AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS "4.2")
     set(GCC_TOO_OLD TRUE)
-  elseif(CMAKE_COMPILER_IS_GNUC AND CMAKE_C_COMPILER_VERSION VERSION_LESS "4.2")
+  elseif(CMAKE_COMPILER_IS_GNUCC AND CMAKE_C_COMPILER_VERSION VERSION_LESS "4.2")
     set(GCC_TOO_OLD TRUE)
   elseif(CMAKE_CXX_COMPILER_ID MATCHES Intel AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS "12.0")
     set(_INTEL_TOO_OLD TRUE)