Inline a small single-use variable

Shrink our CMake build ever so slightly.

Change-Id: I2484305c3cc41432af7d776529e91b257b894658
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/80189
Auto-Submit: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index e9420fd..06ce07e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -116,10 +116,6 @@
   set(CLANG 1)
 endif()
 
-if(CMAKE_SYSTEM_NAME STREQUAL "Emscripten")
-  set(EMSCRIPTEN 1)
-endif()
-
 set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_C_STANDARD 11)
@@ -135,7 +131,7 @@
     # See http://llvm.org/viewvc/llvm-project?view=revision&revision=319116
     set(C_CXX_FLAGS "${C_CXX_FLAGS} -W3 -Wno-unused-parameter -fmsc-version=1900")
   else()
-    if(EMSCRIPTEN)
+    if(CMAKE_SYSTEM_NAME STREQUAL "Emscripten")
       # emscripten's emcc/clang does not accept the "-ggdb" flag.
       set(C_CXX_FLAGS "${C_CXX_FLAGS} -g")
     else()