Allow the case user sets CMAKE_C_COMPILER directly instead of standalone toolchain.

cmake/cross_compiling/android.cmake

@@ -41,7 +41,8 @@ IF(NOT DEFINED ANDROID_STANDALONE_TOOLCHAIN)
         CACHE PATH "Folder holds the standalone toolchain of Android NDK")
 ENDIF()
 IF(NOT ANDROID_STANDALONE_TOOLCHAIN)
-    MESSAGE(FATAL_ERROR "Set ANDROID_STANDALONE_TOOLCHAIN to use a standalone toolchain.\n"
+    MESSAGE(WARNING "It is recommened to set ANDROID_STANDALONE_TOOLCHAIN to "
+            "use a standalone toolchain.\n"
             "To cross-compile for Android, you need to:\n"
             "1. Download an Android NDK from"
             " https://developer.android.com/ndk/downloads/index.html\n"
@@ -65,11 +66,6 @@ ENDIF()
 IF(NOT DEFINED ANDROID_ARM_MODE)
     SET(ANDROID_ARM_MODE ON)
 ENDIF()
-
-IF(NOT DEFINED ANDROID_ARM_NEON)
-    SET(ANDROID_ARM_NEON ON)
-ENDIF()
-
 IF(NOT ANDROID_ARM_MODE OR ANDROID_ARM_MODE STREQUAL "thumb")
     SET(ANDROID_ARM_MODE OFF)
     SET(ANDROID_ARM_MODE_NAME "thumb")
@@ -78,6 +74,10 @@ ELSEIF(ANDROID_ARM_MODE OR ANDROID_ARM_MODE STREQUAL "arm")
     SET(ANDROID_ARM_MODE_NAME "arm")
 ENDIF()
 
+IF(NOT DEFINED ANDROID_ARM_NEON)
+    SET(ANDROID_ARM_NEON ON)
+ENDIF()
+
 IF(${CMAKE_VERSION} VERSION_LESS "3.7.0")  
     IF(${CMAKE_VERSION} VERSION_LESS "3.1.0")
         SET(CMAKE_SYSTEM_NAME "Linux")
@@ -85,6 +85,7 @@ IF(${CMAKE_VERSION} VERSION_LESS "3.7.0")
     MESSAGE(WARNING "It is recommended to use CMake >= 3.7.0 (current version: "
             "${CMAKE_VERSION}), when cross-compiling for Android.")
 
+    IF(ANDROID_STANDALONE_TOOLCHAIN)
         SET(CMAKE_SYSROOT "${ANDROID_STANDALONE_TOOLCHAIN}/sysroot")
 
         IF(NOT CMAKE_SYSTEM_VERSION)
@@ -110,23 +111,45 @@ IF(${CMAKE_VERSION} VERSION_LESS "3.7.0")
             ENDIF()
         ENDIF()
         SET(ANDROID_TOOLCHAIN_PREFIX "${ANDROID_TOOLCHAIN_ROOT}/bin/${ANDROID_TOOLCHAIN_NAME}-")
+    ENDIF()
 
-    IF(EXISTS "${ANDROID_TOOLCHAIN_PREFIX}gcc")
-        SET(CMAKE_C_COMPILER "${ANDROID_TOOLCHAIN_PREFIX}gcc" CACHE PATH "C compiler")
+    # C compiler
+    IF(NOT CMAKE_C_COMPILER)
+        SET(ANDROID_C_COMPILER "${ANDROID_TOOLCHAIN_PREFIX}gcc")
     ELSE()
-        MESSAGE(FATAL_ERROR "Cannot found C compiler: ${ANDROID_TOOLCHAIN_PREFIX}gcc")
+        GET_FILENAME_COMPONENT(ANDROID_C_COMPILER_PATH ${CMAKE_C_COMPILER} PROGRAM)
+        SET(ANDROID_C_COMPILER ${ANDROID_C_COMPILER_PATH})
+    ENDIF()
+    IF(NOT EXISTS ${ANDROID_C_COMPILER})
+        MESSAGE(FATAL_ERROR "Cannot found C compiler: ${ANDROID_C_COMPILER}")
     ENDIF()
 
-    IF(EXISTS "${ANDROID_TOOLCHAIN_PREFIX}g++")
-        SET(CMAKE_CXX_COMPILER "${ANDROID_TOOLCHAIN_PREFIX}g++" CACHE PATH "CXX compiler")
+    # CXX compiler
+    IF(NOT CMAKE_CXX_COMPILER)
+        SET(ANDROID_CXX_COMPILER "${ANDROID_TOOLCHAIN_PREFIX}g++")
     ELSE()
-        MESSAGE(FATAL_ERROR "Cannot found CXX compiler: ${ANDROID_TOOLCHAIN_PREFIX}g++")
+        GET_FILENAME_COMPONENT(ANDROID_CXX_COMPILER_PATH ${CMAKE_CXX_COMPILER} PROGRAM)
+        SET(ANDROID_CXX_COMPILER ${ANDROID_CXX_COMPILER_PATH})
+    ENDIF()
+    IF(NOT EXISTS ${ANDROID_CXX_COMPILER})
+        MESSAGE(FATAL_ERROR "Cannot found CXX compiler: ${ANDROID_CXX_COMPILER}")
     ENDIF()
 
-    IF(EXISTS "${ANDROID_TOOLCHAIN_PREFIX}gfortran")
-        SET(CMAKE_Fortran_COMPILER "${ANDROID_TOOLCHAIN_PREFIX}gfortran" CACHE PATH "Fortran compiler")
+    # Fortran compiler
+    IF(NOT CMAKE_Fortran_COMPILER)
+        SET(ANDROID_Fortran_COMPILER "${ANDROID_TOOLCHAIN_PREFIX}gfortran")
+    ELSE()
+        GET_FILENAME_COMPONENT(ANDROID_Fortran_COMPILER_PATH ${CMAKE_Fortran_COMPILER})
+        SET(ANDROID_Fortran_COMPILER ${ANDROID_Fortran_COMPILER_PATH})
+    ENDIF()
+    IF(NOT EXISTS ${ANDROID_Fortran_COMPILER})
+        SET(ANDROID_Fortran_COMPILER "")
     ENDIF()
 
+    SET(CMAKE_C_COMPILER ${ANDROID_C_COMPILER} CACHE PATH "C compiler" FORCE)
+    SET(CMAKE_CXX_COMPILER ${ANDROID_CXX_COMPILER} CACHE PATH "CXX compiler" FORCE)
+    SET(CMAKE_Fortran_COMPILER ${ANDROID_Fortran_COMPILER} CACHE PATH "Fortran compiler" FORCE)
+
     # Toolchain and ABI specific flags.
     SET(ANDROID_COMPILER_FLAGS "-ffunction-sections -fdata-sections -finline-limit=64")
     SET(ANDROID_LINKER_FLAGS "-Wl,--gc-sections")