Add explicit flags for GMP and MPFR

Bump up arm version to 7

armv7-a has to be added as march, rpi4 fails to build with plain armv7
fixes #3847

deps/GMP/GMP.cmake

@@ -17,11 +17,27 @@ if (MSVC)
     add_custom_target(dep_GMP SOURCES ${_output})
 
 else ()
+    set(_gmp_ccflags "-O2 -DNDEBUG -fPIC -DPIC -Wall -Wmissing-prototypes -Wpointer-arith -pedantic -fomit-frame-pointer -fno-common")
+    set(_gmp_build_tgt "${CMAKE_SYSTEM_PROCESSOR}")
+    if (${CMAKE_SYSTEM_PROCESSOR} MATCHES "arm")
+        set(_gmp_ccflags "${_gmp_ccflags} -march=armv7-a") # Works on RPi-4
+        set(_gmp_build_tgt armv7)
+    endif()
+
+    if (APPLE)
+        set(_gmp_ccflags "${_gmp_ccflags} -mmacosx-version-min=${DEP_OSX_TARGET}")
+        set(_gmp_build_tgt "--build=${_gmp_build_tgt}-apple-darwin")
+    elseif(CMAKE_SYSTEM_NAME STREQUAL "Linux")
+        set(_gmp_build_tgt "--build=${_gmp_build_tgt}-pc-linux-gnu")
+    else ()
+        set(_gmp_build_tgt "") # let it guess
+    endif()
+
     ExternalProject_Add(dep_GMP
         # URL  https://gmplib.org/download/gmp/gmp-6.1.2.tar.bz2
         URL https://gmplib.org/download/gmp/gmp-6.2.0.tar.lz
         BUILD_IN_SOURCE ON 
-        CONFIGURE_COMMAND ./configure --enable-shared=no --enable-cxx=yes --enable-static=yes "--prefix=${DESTDIR}/usr/local" --with-pic
+        CONFIGURE_COMMAND  env "CFLAGS=${_gmp_ccflags}" "CXXFLAGS=${_gmp_ccflags}" ./configure --enable-shared=no --enable-cxx=yes --enable-static=yes "--prefix=${DESTDIR}/usr/local" ${_gmp_build_tgt}
         BUILD_COMMAND     make -j
         INSTALL_COMMAND   make install
     )

deps/MPFR/MPFR.cmake

@@ -21,9 +21,9 @@ else ()
     ExternalProject_Add(dep_MPFR
         URL http://ftp.vim.org/ftp/gnu/mpfr/mpfr-3.1.6.tar.bz2 https://www.mpfr.org/mpfr-3.1.6/mpfr-3.1.6.tar.bz2  # mirrors are allowed
         BUILD_IN_SOURCE ON
-        CONFIGURE_COMMAND ./configure --prefix=${DESTDIR}/usr/local --enable-shared=no --enable-static=yes --with-gmp=${DESTDIR}/usr/local --with-pic
+        CONFIGURE_COMMAND env "CFLAGS=${_gmp_ccflags}" "CXXFLAGS=${_gmp_ccflags}" ./configure --prefix=${DESTDIR}/usr/local --enable-shared=no --enable-static=yes --with-gmp=${DESTDIR}/usr/local ${_gmp_build_tgt}
         BUILD_COMMAND make -j
         INSTALL_COMMAND make install
         DEPENDS dep_GMP
     )
-endif ()
+endif ()