Atmel Studio 7 Error: recipe for target 'main.o' failed

------ Build started: Project: FS1.2, Configuration: Release AVR ------

Build started.

Project "FS1.2.cproj" (default targets):

Target "PreBuildEvent" skipped, due to false condition; ('$(PreBuildEvent)'!='') was evaluated as (''!='').

Target "CoreBuild" in file "C:\Program Files (x86)\Atmel\Studio\7.0\Vs\Compiler.targets" from project "D:\Projekte\FS1.2\FS1.2.cproj" (target "Build" depends on it):

  Using "RunCompilerTask" task from assembly "C:\Program Files (x86)\Atmel\Studio\7.0\Extensions\Application\AvrGCC.dll".

  Task "RunCompilerTask"

    Shell Utils Path C:\Program Files (x86)\Atmel\Studio\7.0\shellUtils

    C:\Program Files (x86)\Atmel\Studio\7.0\shellUtils\make.exe all --jobs 2 --output-sync 

    Building file: .././main.c

    Invoking: AVR/GNU C Compiler : 4.9.2

    "C:\Program Files (x86)\Atmel\Studio\7.0\toolchain\avr8\avr8-gnu-toolchain\bin\avr-gcc.exe"  -x c -funsigned-char -funsigned-bitfields -DNDEBUG -DF_CPU=8000000UL  -I"C:\Program Files (x86)\Atmel\Studio\7.0\Packs\atmel\ATmega_DFP\1.1.130\include"  -Os -ffunction-sections -fdata-sections -fpack-struct -fshort-enums -Wall -mmcu=atmega88pa -B "C:\Program Files (x86)\Atmel\Studio\7.0\Packs\atmel\ATmega_DFP\1.1.130\gcc\dev\atmega88pa" -c -std=gnu99 -MD -MP -MF "main.d" -MT"main.d" -MT"main.o"   -o "main.o" ".././main.c" 

D:\Projekte\FS1.2\Release\Makefile(156,1): error: recipe for target 'main.o' failed

    .././main.c: In function 'main':

D:\Projekte\FS1.2\main.c(463,3): warning: implicit declaration of function 'clock_prescale_set' [-Wimplicit-function-declaration]

       clock_prescale_set(clock_div_2);

       ^

D:\Projekte\FS1.2\main.c(463,22): error: 'clock_div_2' undeclared (first use in this function)

       clock_prescale_set(clock_div_2);

                          ^

D:\Projekte\FS1.2\main.c(463,22): info: each undeclared identifier is reported only once for each function it appears in

D:\Projekte\FS1.2\main.c(477,22): error: 'clock_div_1' undeclared (first use in this function)

       clock_prescale_set(clock_div_1);

                          ^

D:\Projekte\FS1.2\main.c(173,7): warning: variable 'fReqAck' set but not used [-Wunused-but-set-variable]

      bool fReqAck = true;

           ^

    make: *** [main.o] Error 1

    make: *** Waiting for unfinished jobs....

    Building file: .././com.c

    Invoking: AVR/GNU C Compiler : 4.9.2

    "C:\Program Files (x86)\Atmel\Studio\7.0\toolchain\avr8\avr8-gnu-toolchain\bin\avr-gcc.exe"  -x c -funsigned-char -funsigned-bitfields -DNDEBUG -DF_CPU=8000000UL  -I"C:\Program Files (x86)\Atmel\Studio\7.0\Packs\atmel\ATmega_DFP\1.1.130\include"  -Os -ffunction-sections -fdata-sections -fpack-struct -fshort-enums -Wall -mmcu=atmega88pa -B "C:\Program Files (x86)\Atmel\Studio\7.0\Packs\atmel\ATmega_DFP\1.1.130\gcc\dev\atmega88pa" -c -std=gnu99 -MD -MP -MF "com.d" -MT"com.d" -MT"com.o"   -o "com.o" ".././com.c" 

    Finished building: .././com.c

  Done executing task "RunCompilerTask" -- FAILED.

Done building target "CoreBuild" in project "FS1.2.cproj" -- FAILED.

Done building project "FS1.2.cproj" -- FAILED.

Build FAILED.

========== Build: 0 succeeded or up-to-date, 1 failed, 0 skipped ==========